US experts say North Korea appears to be upgrading one of its two rocket launch sites, perhaps in a move to test bigger rockets.
“Important progress” had been made at Tonghae Satellite Launching Ground since October 2012, the analysis from the 38 North website said.
Activities around the new launch pad also revealed possible evidence of assistance from Iran, it said.
Pyongyang used a three-stage rocket to put a satellite into space last year.
That launch – condemned by the UN as a banned test of missile technology – took place at the Sohae launch site.
But previous unsuccessful attempts in 2006 and 2009 took place at the Tonghae site, which is also known as Musudan-ri.
The analysis from 38 North, the website of the US-Korea Institute at Johns Hopkins School of Advanced International Studies in Baltimore, was based on satellite imagery.
Construction of the new launch pad was continuing, it said, with images showing Pyongyang would be able to test rockets “perhaps three to four times the size of the Unha [launched in December 2012] when construction is completed, possibly in 2016”.
Two new design features were similar to those used at the Semnan Launch Complex in Iran, it said.
The images also confirmed activity at the old launch pad.
US experts say North Korea appears to be upgrading one of its two rocket launch sites, perhaps in a move to test bigger rockets
“That activity may be related to another round of modifications intended to support future launches of the Unha rocket or possibly another liquid-fuelled missile,” 38 North said, while cautioning that more information was needed.
North Korea last week conducted its third nuclear test, claiming to have successfully detonated a smaller but more powerful device than in previous tests.
The move drew immediate condemnation from the UN Security Council.
Observers fear North Korea is working towards creating a nuclear device small enough to fit on a long-range missile.
North Korea has reacted angrily to a UN resolution condemning its recent rocket launch, pledging to strengthen military and nuclear capabilities.
The Security Council unanimously passed the resolution, which also expanded existing sanctions, on Tuesday.
North Korea, in a statement early on Wednesday, pledged to bolster its “nuclear deterrent” and ruled out denuclearization talks.
The resolution followed Pyongyang’s successful December launch.
North Korea said the three-stage rocket put a communications satellite into space.
But its neighbors and the US said the move constituted a test of long-range missile technology banned under UN resolutions passed after Pyongyang’s nuclear tests in 2006 and 2009.
The Security Council resolution was proposed by the US and backed by China, North Korea’s closest ally and biggest trading partner.
It represented a compromise between the two nations, with the US pushing for new sanctions and China for a statement, rather than a resolution. Chinese support was a blow for Pyongyang, observers say.
North Korea has reacted angrily to a UN resolution condemning its recent rocket launch, pledging to strengthen military and nuclear capabilities
Under the resolution – which pledged “significant action” if North Korea carried out a third nuclear test – North Korea’s space agency, a bank and a number of trading companies and individuals were added to existing sanctions lists.
“This resolution demonstrates to North Korea that there are unanimous and significant consequences for its flagrant violation” of previous resolutions, said US Ambassador to the UN Susan Rice.
Both South Korea and Japan welcomed the resolution, with Japanese Prime Minister Shinzo Abe calling it a “resolute action”.
Chinese envoy to the UN Li Baodong, meanwhile, said Beijing believed “that action taken by the council should be prudent, measured, proportionate and conducive to stability”.
North Korea responded swiftly, with a statement from its foreign minister carried by state news agency KCNA condemning the “extremely unfair” resolution as a violation of sovereign rights.
“We will take physical actions aimed at expanding and strengthening our self-defensive military forces, including nuclear deterrence,” it said.
It also stated that there would be “no dialogue to discuss denuclearization of the Korean peninsula”, in an apparent reference to long-stalled six-nation talks aimed at ending Pyongyang’s nuclear ambitions.
In recent weeks reports from both the US and South Korea have described activity at North Korea’s nuclear test sites, sparking concerns North Korea could be preparing for a third test.
Pyongyang has conducted two nuclear tests, the first in 2006 and the second in 2009.
The US and North Korea’s neighbors fear Pyongyang’s ultimate goal is to put a nuclear warhead on a long-range missile that could target the west coast of the US, but it is not believed to have mastered the technology yet.
The US is moving navy ships into position to track North Korea’s rocket due to launch later this month.
The warships were moved to achieve “the best situational awareness”, the US military chief in the region said.
Japan’s government, meanwhile, has formally issued an order to its military to shoot down any rocket debris that infringes on its territory.
North Korea plans to launch its rocket between 10 and 22 December, saying it will put a satellite into space.
The US and other nations say the launch constitutes a test of long-range missile technology banned under UN resolutions.
North Korea conducted a similar launch in April 2012, but the rocket flew only for a short time before crashing into waters off the Korean peninsula.
This launch window includes two key dates – 17 December marks the first anniversary of the death of North Korean leader Kim Jong-il and 19 December is when South Korea’s presidential election takes place.
South Korea’s Yonhap news agency – citing unidentified sources – reports that all three stages of the rocket are now in place at the launch site and that fuel is being injected into a storage tank, after which the rocket will be fuelled.
But a US think-tank says preparations may not be this far advanced, citing satellite images of the launch site.
The US is moving navy ships into position to track North Korea’s rocket due to launch later this month
Snow had forced a temporary halt to work on 4 December, the US-Korea Institute at Johns Hopkins University said on its 38 North website, but the North Koreans still had time to complete preparations on schedule.
Based on co-ordinates provided by Pyongyang, the rocket is expected to fly south – with stages dropping into the sea west of the Korean peninsula and then east of the Philippines.
Admiral Samuel Locklear, head of US forces in the Asia-Pacific region, said preparations for the launch were being watched “very closely”.
On the warships, he said it “should seem logical that we’ll move them around so we have the best situational awareness”.
“To the degree that those [navy] ships are capable of participating in ballistic missile defense, then we will position them to be able to do that,” he said.
The rocket’s flight plan also takes it close to parts of Japan’s Okinawa prefecture. The Japanese government has pledged to shoot down any debris that falls over its territory.
It is deploying three warships equipped with missile interceptors and is also positioning PAC-3 missile interceptors on the ground at four locations in Okinawa prefecture, Kyodo news agency said.
All Nippon Airways, meanwhile, says it is adjusting flight plans to avoid the area off the Philippines where the second stage of the rocket may fall.
Similar preparations by the US and Japanese militaries were seen ahead of the failed launch in April. But the 30 m (100 ft) Unha-3 rocket is thought to have flown for only minutes before breaking up.
North Korea has not yet successfully launched a three-stage rocket, despite four attempts since 1998. It is believed to be working on the development of a long-range missile capable of reaching the west coast of the US mainland.
The US and its allies say the rocket launches represent banned tests of ballistic missile technology because the basic technology is the same.