North Korea has boosted defences on its east coast, a South Korean lawmaker said on Tuesday, after the North said US President Donald Trump had declared war and that it would shoot down US bombers flying near the peninsula.
Tensions have escalated since North Korea conducted its sixth and most powerful nuclear test on September 3, but the rhetoric has reached a new level in recent days with leaders on both sides exchanging threats and insults.
North Korean Foreign Minister Ri Yong Ho said Trump's Twitter comments, in which the US leader said Ri and leader Kim Jong Un "won't be around much longer" if they acted on their threats, amounted to a declaration of war and that Pyongyang had the right to take countermeasures.
South Korean lawmaker Lee Cheol-uoo, briefed by the country's spy agency, said the reclusive North was in fact bolstering its defences by moving aircraft to its east coast and taking other measures after US bombers flew close to the Korean peninsula at the weekend.
Lee said the United States appeared to have disclosed the flight route of the bombers intentionally because North Korea seemed to be unaware.
Ri, the foreign minister, said on Monday the North's right to countermeasures included shooting down US bombers "even when they are not inside the airspace border of our country".
"The whole world should clearly remember it was the US who first declared war on our country," he told reporters in New York on Monday, where he had been attending the annual UN General Assembly.
"The question of who won't be around much longer will be answered then," he said.
White House spokeswoman Sarah Sanders denied on Monday that the United States had declared war, calling the suggestion "absurd".
South Korean President Moon Jae-in urged Kim Jong Un to resume military talks and reunions of families split by the 1950-53 Korean War to ease tension.
"Like I've said multiple times before, if North Korea stops its reckless choices, the table for talks and negotiations always remains open," said Moon.
He was speaking at an event to mark an October 4, 2007, summit declaration promoting goodwill signed between then-South Korean president Roh Moo-hyun and his North Korean counterpart, Kim Jong Il, Kim Jong Un's father.
US 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.
North Korea has been working to develop nuclear-tipped missiles capable of hitting the US mainland, which Trump has said he will never allow.
The US and South Korea are technically still at war with North Korea after the 1950-53 conflict ended in a truce and not a peace treaty.
The September 3 nuclear test prompted a new round of sanctions on North Korea after the Security Council voted unanimously on a resolution condemning the test.
The North says it needs its weapons programmes to guard against US invasion and regularly threatens to destroy the US, South Korea and Japan.
However, the rhetoric has been ratcheted up well beyond normal levels, raising fears that a miscalculation by either side could have massive repercussions.
Trump's threat last week to totally destroy North Korea, a country of 26 million people, if it threatened the US or its allies, prompted an unprecedented direct statement by Kim, calling Trump a "mentally deranged US dotard" and vowing to tame the US threat with fire.