The US has said a statement from North Korea accusing Washington of declaring war on it was “absurd”.
The White House also warned Pyongyang 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.
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 Monday that “the whole world should clearly remember it was the US who first declared war on our country”.
His 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.
Two days ago 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, Mr Ri said his country had the right to shoot down US warplanes even if they were not in North Korea’s airspace.
Later on Monday, 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.”
‘Declarations of war’
North Korea frequently calls foreign action against it “a declaration of war”. Here are just a few of the occasions it has used the expression:
July 2016: When US imposed sanctions on Kim Jong-un and several other officials over human rights abuses – North Korea said that was “a hideous crime”.
February 2016: When South Korea withdrew from the shared economic zone at Kaesong – North Korea said this was “an end to the last lifeline” of inter-Korean relations.
October 2014: It said if balloons carrying anti-Pyongyang leaflets were allowed to drift across the border from the South it would be “a de facto declaration of war.
October 2006: It said UN Security Council sanctions passed in response to North Korea’s first nuclear test meant a war declaration.
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, 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 Pyongyang was readjusting the position of its military aircraft and strengthening its coastal defences, 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”.
“We want things to calm down,” China’s ambassador to the UN, Liu Jieyi, told Reuters. “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.
It says nuclear capabilities are its only deterrent against an outside world seeking to destroy it.
After the North’s latest and most powerful nuclear test earlier this month, the UN Security Council approved new sanctions on the country.