North Korea appears to have boosted defenses on its east coast, South Korea's Yonhap news agency said on Tuesday, after the North said U.S. President Donald Trump had declared war and that it would shoot down U.S. bombers flying near the peninsula.

Tensions have escalated since North Korea conducted its sixth and most powerful nuclear test on Sept. 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 U.S. 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 suggested the reclusive North was, in fact, bolstering its defenses by moving aircraft to its east coast and taking other measures after U.S. 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 countermeasures included shooting down U.S. bombers "even when they are not inside the airspace border of our country."

"The whole world should clearly remember it was the U.S. 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."

Speaking in Beijing, Chinese Foreign Ministry spokesman Lu Kang said war on the Korean peninsula would have no winner.

"We hope the U.S. and North Korean politicians have sufficient political judgment to realize that resorting to military force will never be a viable way to resolve the peninsula issue and their own concerns," Lu told a daily news briefing.

"We also hope that both sides can realize that being bent on assertiveness and provoking each other will only increase the risk of conflict and reduce room for policy maneuvers. War on the peninsula will have no winner."

While repeatedly calling for dialogue to resolve the issue, China has also signed up for increasingly tough U.N. sanctions against North Korea.

China's fuel exports to North Korea fell in August, along with iron ore imports from the isolated nation, as trade slowed after the latest U.N. sanctions, but coal shipments resumed after a five-month hiatus, customs data showed 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.

During a visit to India, U.S. Defense Secretary James Mattis said diplomatic efforts to deal with the crisis were continuing.

"You have seen unanimous United Nations Security Council resolutions passed that have increased the pressure, economic pressure, and diplomatic pressure, on the North, and at the same time, we maintain the capability to deter North Korea's most dangerous threats," he told reporters in the Indian capital.