The US has denied a war declaration against North Korea.
A statement from North Korea on September 25 accuses Washington of declaring war.
The White House also warned North Korea to stop provocations after it said it had the right to shoot down US bombers.
A UN spokesman said fiery talk could lead to fatal misunderstandings.
Meanwhile, South Korea has called for a level-headed response, warning that accidental clashes in the region could quickly spiral out of control.
North Korean Foreign Minister Ri Yong-ho told reporters on September 25 that “the whole world should clearly remember it was the US who first declared war on our country”.
Ri Yong-ho’s comments were a response to a tweet from President Donald Trump suggesting North Korea would not “be around much longer” if its leaders continued their rhetoric.
On September 24, US warplanes flew close to North Korea’s coast in a show of force.
Speaking as he left New York after the UN General Assembly, Ri Yong-ho said his country had the right to shoot down US warplanes even if they were not in North Korea’s airspace.
On September 25, White House spokeswoman Sarah Huckabee Sanders said the US had “not declared war against North Korea and frankly the suggestion of that is absurd”.
Pentagon spokesman Col. Robert Manning reacted by saying: “If North Korea does not stop their provocative actions, you know, we will make sure that we provide options to the president to deal with North Korea.”
South Korea – technically at war with North Korea since the 1950s – called for “astuteness and steadfastness” in responding to what it describes as continued provocations by Pyongyang.
Speaking in New York, South Korean Foreign Minister Kang Kyung-wha called for the prevention of any “further escalation of tensions, or any kind of accidental military clashes in the region which can quickly spiral out of control”.
South Korea’s intelligence service said North Korea was readjusting the position of its military aircraft and strengthening its coastal defenses, according to the South’s news agency Yonhap.
Stéphane Dujarric, a spokesman for UN Secretary General António Guterres, said that “fiery talk can lead to fatal misunderstandings” and that “the only solution for this is a political solution”.
China’s ambassador to the UN, Liu Jieyi, told Reuters: “We want things to calm down.
“It’s getting too dangerous and it’s in nobody’s interest.”
Despite weeks of tension, experts have played down the risk of direct conflict.
North Korea has continued to carry out nuclear and ballistic missile tests in recent weeks, in defiance of successive rounds of UN sanctions.
Pyongyang says nuclear capabilities are its only deterrent against an outside world seeking to destroy it.
After North Korea’s latest and most powerful nuclear test earlier this month, the UN Security Council approved new sanctions on the country.