According to South Korean officials, a North Korean patrol boat violated a sea border with the South several times late on Monday.
The ship spent a few hours south of the border, nearing a South Korean border island, before returning after repeated warnings from the South, they added.
North Korea disputes the maritime border and has sent boats across it in the past.
The incident comes amid joint military drills between the US and South Korea which are opposed by Pyongyang.
The North Korean patrol ship crossed the Northern Limit Line (NLL), which South Korea considers the maritime border between the two sides, at around 22:46 local time, South Korea’s defense ministry said.
The South broadcast warnings 10 times before the ship returned, at around 02:25 local time, the ministry added.
A North Korean patrol boat violated a sea border with the South several times late on Monday
The NLL was drawn unilaterally by the United Nations Command at the end of the 1950-1953 Korean War.
However, North Korea has disputed the NLL and drawn its own border further south of the line.
“The North Korean ship’s NLL violation is seen as part of military drills,” defense ministry spokesman Kim Min-seok said.
“It is believed that [the ship] intended to test the South Korean military.”
The ship was said to have come within 15 miles of South Korea’s border island of Baengnyeong.
Monday’s incident comes on the same day joint annual US-South Korea military exercises began.
More than 12,500 US troops will take part in the exercises, which include Key Resolve, a computer-based simulation, and Foal Eagle, which involves air, ground and naval drills.
Pyongyang is opposed to the drills and has previously called them “exercises of war”.
Tuesday is also the last day of rare family reunions for North and South Korean relatives separated after the Korean War.
South Korea and US joint military drills have begun, amid rare family reunions between North and South Korean relatives separated for decades.
The annual military exercises will last until April 18.
They will involve Key Resolve, a computer-based simulation, and Foal Eagle, which involves air, ground and naval drills.
Pyongyang is opposed to the drills and had previously threatened to cancel the reunions if the exercises went ahead.
The military drills will involve more than 12,500 US troops.
The US and South Korea describe the annual drills as defensive in nature, but Pyongyang has described them as “exercises of war”.
Last year, the exercises led to a prolonged surge in tensions, with North Korea threatening pre-emptive nuclear strikes and attacks on South Korean and US targets.
The rhetoric this year has so far been relatively mild, but the drills are scheduled to last until April, and many here see them as the toughest test yet of whether ties between the two Koreas are warming.
South Korea and US annual military exercises will last until April 18
Speaking on Monday, South Korean Defense Ministry spokesman Kim Min-seok said: “As of now, there are no unusual movements from North Korea. We will only take action against North Korea if it makes provocations or denunciations.”
Also on Monday, around 360 South Koreans met their North Korean relatives for the first time since the 1950-1953 Korean War, at a family reunion event in North Korea’s Mount Kumgang resort.
They were the second set of relatives chosen to attend the reunions, which come amid an apparent thaw in inter-Korean ties.
Many people were separated from their relatives by the division of the Korean peninsula after the Korean War.
Pyongyang has been accused of using the family reunions, which are highly emotional events, as a bargaining chip.
North Korea has in the past canceled the reunions after the South took actions it opposed – most recently in September.
Meanwhile, South Korea also offered to assist the North with tackling an outbreak of foot-and-mouth disease.
North Korean state media said that thousands of pigs had been affected by the disease.
South Korea, which has also been hit by outbreaks before, has offered to send aid, including medical goods and vaccines, officials said.
South Korea and the United States have begun a new round of joint military exercises on Monday, officials say, the second round to take place this year.
Thousands of soldiers are taking part in computer-aided drills designed to test defense capabilities.
South Korea says that North Korea, which was angered by the previous drill, was notified well in advance.
The drills come as the North agreed to family reunions and reached a deal with the South on an industrial zone.
The drills, known as Ulchi Freedom Guardian, are expected to last for 12 days and mobilizes about 50,000 members of the South Korean military and 30,000 US servicemen, reports Yonhap news agency.
South Korea and the US have begun a new round of joint military exercises on Monday, the second to take place this year
They are intended to help “ensure stability and security on the peninsula and reaffirm the US commitment to the north-east Asia region”, says a statement from the United States Forces Korea.
The drills come as tensions are starting to ease in the peninsula.
On Sunday, North Korea agreed to a South Korean proposal to resume in September reuniting families separated since the 1950-1953 Korean War.
Many families were separated at the end of the war by the dividing of the peninsula. The two sides remain technically at war, because the conflict ended in an armistice and not a peace deal. The last reunions were held in 2010.
Last week, officials of the two Koreas also reached an agreement about re-opening the Kaesong joint industrial zone – the last functioning inter-Korean joint project and a key source of revenue for Pyongyang.
The Kaesong Industrial Complex, which lies just inside North Korea, is home to 123 South Korean factories which employ more than 50,000 North Korean workers.
North Korea withdrew its workers in April, angered by the expansion of UN sanctions after its February 12 nuclear test and annual US-South Korea military drills.
The accord came after six rounds of talks ended unsuccessfully.
The United States and South Korea have begun annual military drills amid high tensions with North Korea following the UN sanctions vote.
Pyongyang has strongly condemned the exercises, threatening to scrap the armistice that ended the Korean War.
Seoul says North Korea also appears to have carried out a threat made last week to sever a cross-border hotline.
The drills come days after the UN approved new sanctions on North Korea following its nuclear test in February.
The test last month was the communist country’s third. It followed an apparently successful launch in December of a three-stage rocket, seen as a banned test of missile technology.
North Korea’s neighbors and the US fear it is working to build a nuclear warhead small enough to put on a missile, but believe it does not yet have the capabilities to do so.
The US-South Korea joint drills, which are known as “Key Resolve”, last two weeks and involve more than 13,000 troops. Another joint exercise, known as Foal Eagle, has been under way since the beginning of March.
Both exercises take place every year, usually prompting strong rhetoric from the North.
The US and South Korea have begun annual military drills amid high tensions with North Korea following the UN sanctions vote
In apparent response to the UN sanctions vote, however, North Korea has issued multiple threats, promising to abandon the Korean War armistice, pull out of non-aggression pacts with North Korea and cut cross-border links, including the hotline.
Early on Monday, South Korea’s Unification Ministry said efforts to contact the North via the hotline had been unsuccessful. The hotline, installed in 1971, has been severed on five previous occasions.
North Korea also appears set to carry out its own military drills this week, South Korea says.
Late last week, South Korea’s new President, Park Geun-hye, warned that the security situation on the Korean Peninsula was “very grave”.
The two Koreas remain technically at war, because an armistice was signed at the end of the 1950-53 Korean conflict, rather than a peace deal.
Tensions have boiled over on a number of occasions in the past, most recently in November 2010 when four South Koreans were killed in North Korean shelling on a border island.
North Korea’s Communist Party newspaper, Rodong Sinmun, warned on Monday that the situation was unpredictable.
“With the ceasefire agreement blown apart… no one can predict what will happen in this land from now on,” the mouthpiece said.
North Korea is holding large-scale military drills amid heightened tensions on the peninsula, South Korea says.
The move comes after the North Korea threatened to scrap the 60-year truce which ended the Korean War.
Later on Thursday, the UN Security Council will vote on a resolution imposing tighter sanctions on Pyongyang following its recent nuclear test.
Meanwhile, Australia has put on hold plans to reopen a North Korea embassy in its capital, Canberra.
Patrick Low, a spokesman for the Australian foreign ministry, said there was still “some merit” in having a North Korean embassy in Australia, including enabling more direct communication on human rights issues.
But said the plans had been frozen “until further notice” while Australia worked with the UN Security Council on its response to North Korea’s nuclear test.
North Korea first opened an embassy in Australia in 2002, but closed it in 2008 for financial reasons.
The nuclear test, North Korea’s third, followed its apparently successful launch in December of a three-stage rocket – a move condemned by the UN as a banned test of missile technology.
North Korea is holding large-scale military drills amid heightened tensions on the peninsula
Pyongyang claims its nuclear test involved a smaller and more powerful device – prompting concerns it could be moving closer to creating a warhead small enough to arm a missile.
A spokesman for the South Korean Defence Ministry, Kim Min-seok, said Pyongyang was “currently conducting various drills involving the army, navy and air force,” and that further exercises were being prepared, the Yonhap news agency reports.
“Given that this training can [be] extended into a provocation at any time, we are taking great interest in these activities, and are strengthening our own preparedness as well,” he said.
South Korea has previously warned it will respond to any provocation from its northern neighbor, with whom it remains technically at war after the Korean War ended in 1953 with an armistice, not a formal peace treaty.
On Tuesday, North Korea’s military command said it would end that armistice on March 11 because of the threat of sanctions and the joint South Korea-US military drills, which take place annually but which Pyongyang sees as war preparation.
The UN Security Council, meanwhile, will vote on the sanctions resolutions on North Korea at 10:00 EST.
The resolution, which is expected to pass, was proposed by China and the US, and will target North Korea’s diplomats, cash transfers and access to luxury goods.
It will also impose asset freezes and travel bans on three individuals and two corporations linked to North Korea’s military.
The sanctions have been described by Washington’s UN ambassador Susan Rice as “some of the toughest sanctions” the UN had ever imposed.