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.

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.

North Korea has been working to develop nuclear-tipped missiles capable of hitting the U.S. mainland, which Trump has said he will never allow.

The United States and South Korea are technically still at war with North Korea after the 1950-53 Korean conflict ended in a truce and not a peace treaty.

The Sept. 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 programs to guard against U.S. invasion and regularly threatens to destroy the United States, 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 United States or its allies, prompted an unprecedented direct statement by Kim, calling Trump a “mentally deranged U.S. dotard” and vowing to tame the U.S. threat with fire.

White House National Security Adviser H.R. McMaster defended Trump’s rhetoric and said on Monday he agreed that the risk was that Kim might fail to realize the danger he and his country faced.

However, McMaster also acknowledged the risks of escalation with any U.S. military option.

“We don’t think there’s an easy military solution to this problem,” said McMaster, who believed any solution would be an international effort.

“There’s not a precision strike that solves the problem. There’s not a military blockade that can solve the problem,” McMaster said.