North Korea has reportedly been boosting defences on its east coast after claiming US President Donald Trump had declared war and that it would shoot down US bombers flying nearby.
Tensions have escalated on the Korean peninsula 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.
Yonhap news agency suggested 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.
The unverified Yonhap report said the United States appeared to have disclosed the flight route of the bombers intentionally because North Korea seemed to be unaware. South Korea's National Intelligence Service was unable to confirm the report immediately.
Ri said on Monday the North's right to counter-measures 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 United Nations 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".
Chinese Foreign Ministry spokesman Lu Kang said Beijing hopes US and North Korean politicians realise that resorting to military means would never be a viable way out.
China also disapproved of the escalation of the war of words between the United States and North Korea, Lu told a daily news briefing in Beijing on Tuesday.
In Moscow, Russia's Foreign Ministry said it was working behind the scenes to find a political solution and that using sanctions against North Korea was almost exhausted.
US Defence Secretary James Mattis, speaking during a visit to India, said he appreciated global efforts to increase pressure on North Korea for its dangerous behaviour.