North Korea's foreign minister said on Monday that U.S. President Donald Trump had declared war on North Korea and that Pyongyang reserved the right to take countermeasures, including shooting down U.S. bomber planes even if they are not in its airspace.

Ri Yong-ho said a Twitter message by Trump on Saturday, in which the president warned that the minister and North Korean leader Kim Jong-un "won't be around much longer" if they acted on their threats, amounted to a declaration of war.

White House spokesperson Sarah Huckabee Sanders on Monday denied the United States had declared war, calling the suggestion "absurd."

Speaking earlier in New York, where he had been attending the annual UN General Assembly, Ri told reporters: "The whole world should clearly remember it was the U.S. who first declared war on our country.

"We will have every right to make countermeasures, including the right to shoot down United States strategic bombers even when they are not inside the airspace border of our country," Ri said.

"The question of who won't be around much longer will be answered then," he added.

On Saturday, U.S. air force B-1B Lancer bombers escorted by fighter jets flew east of North Korea in a show of force after a heated exchange of rhetoric between Trump and Kim over North Korea's nuclear and missile programs.

US North Korea

On Saturday, U.S. Air Force B-1B Lancer bombers escorted by fighter jets flew east of North Korea in a show of force. (Senior Airman John Linzmeier/U.S. Air Force/Associated Press)

"That operation was conducted in international airspace, over international waters, so we have the right to fly, sail and operate where legally permissible around the globe," Pentagon spokesperson Col. Robert Manning said on Monday.

North Korea, which has remained technically at war with the United States since the 1950-53 Korean War ended in a truce and not a peace treaty, has been working to develop nuclear-tipped missiles capable of hitting the U.S. mainland, and conducted its sixth and largest nuclear test this month.

Fears of miscalculation

Pyongyang, which has pursued its missile and nuclear programs in defiance of international sanctions, accuses the United States of planning to invade and regularly threatens to destroy it and its Asian allies.

However, recent rhetoric from both sides has been unusually harsh, raising fears of miscalculation that could have massive repercussions, even though U.S. officials have repeatedly stressed the administration prefers a negotiated solution.

The latest round of heavy verbal salvoes began when Trump, in his maiden UN address last Tuesday, threatened to "totally destroy" North Korea, a country of 26 million people, if it threatened the U.S. or its allies.

Kim Jong-un and Donald Trump trade insults amid escalating U.S.- North Korea tensions2:13

In an unprecedented direct statement on Friday, Kim called Trump a "mentally deranged U.S. dotard" he would tame with fire.

Kim said North Korea would consider the "highest level of hardline countermeasure in history" against the U.S., and said Trump's comments confirmed Pyongyang's nuclear program was "the correct path."

Ri told the UN General Assembly on Saturday that targeting the U.S. mainland with its rockets was inevitable after "Mr. Evil President" Trump called Kim a "rocket man … on a suicide mission."

'They won't be around much longer!'

On Twitter late Saturday, Trump replied: "Just heard Foreign Minister of North Korea speak at UN. If he echoes thoughts of Little Rocket Man, they won't be around much longer!"

On Monday, White House National Security Adviser H.R. McMaster defended Trump's rhetoric, saying he agreed with the U.S. president that the risk was that Kim Jong-un might fail to realize the danger he and his country were facing.

McMaster voiced confidence that the U.S. could, for example, impose a military blockade if it chose, perhaps even as part of a multinational effort. But he acknowledged risks of escalation with any U.S. military option.

"We don't think there's an easy military solution to this problem. There's not a precision strike that solves the problem. There's not a military blockade that can solve the problem," he said, adding that ultimately it would come down to an international effort.

North Korea's foreign minister blasts Donald Trump during UN speech1:58

Still, McMaster told a conference hosted by the Institute for the Study of War that Washington was concerned a nuclear-armed North Korea capable of hitting the U.S. was likely to engage in "nuclear blackmail," for instance to try to achieve its goal of getting U.S. troops off the Korean peninsula.

South Korean Foreign Minister Kang Kyung-wha said it was vital Seoul and the U.S. handle the situation "with astuteness and steadfastness ... to prevent a further escalation of tension or any kind of accidental military clashes" in the region.

"There cannot be another outbreak of war on the Korean peninsula; the consequences would be devastating," she told Washington's Center for Strategic and International Studies.        

Calls for restraint

Former U.S. secretary of state Madeleine Albright, the most senior serving U.S. official ever to visit Pyongyang, said it was "important to lower the temperature" of rhetoric.

"I'm kind of concerned about accidents of some kind that might happen," she said.

UN Secretary General Antonio Guterres said the only solution to the crisis was a political one.

US South Korea

South Korean Foreign Minister Kang Kyung-wha, left, with former U.S. Secretary of State Madeleine Albright, at a forum in Washington, Monday, said it was vital Seoul and the U.S. 'prevent a further escalation of tension or any kind of accidental military clashes in the region.' (Manuel Balce Ceneta/Associated Press)

"Fiery talk can lead to fatal misunderstandings," UN spokesman Stephane Dujarric told reporters.

China, North Korea's neighbour and main ally which has nevertheless backed UN sanctions over Pyongyang's nuclear program, called for restraint on all sides.

"We want things to calm down. It's getting too dangerous and it's in nobody's interest," China's UN ambassador, Liu Jieyi, told Reuters.

"We certainly hope [the U.S. and North Korea] will see that there is no other way than negotiations to solve the nuclear issue on the Korean Peninsula.... The alternative is a disaster."

China said it was vital that everyone implement all North Korea-related UN resolutions, which call for both tighter sanctions and efforts to resume dialogue.

Chinese state media said President Xi Jinping told British Prime Minister Theresa May by telephone that he hoped the U.K. could play a constructive role in achieving a peaceful solution via talks.

Defence experts said North Korea would have difficulty shooting down a U.S. bomber with missiles or fighter planes given its limited capabilities. And if it tried and failed, it would appear weak.

"It is unlikely to take such a risk," said Bruce Bennett of the Rand Corp think-tank. "So this sounds like another attempt by North Korea to 'deter by bluster' U.S. actions the regime does not like."

Ri warned on Friday North Korea might test a hydrogen bomb over the Pacific Ocean in what would be North Korea's first atmospheric nuclear test. Experts said such a move, while perhaps not imminent, would be proof of North Korea's ability to successfully deliver a nuclear warhead on a missile.