A nuclear-powered U.S. submarine made a port call in South Korea on Tuesday in a show of force amid concerns that North Korea may mark the foundation of its military with a missile launch or a nuclear test, defying U.S. and Chinese pressure.

The port call by the USS Michigan, announced by the U.S. military in South Korea, came as the top nuclear envoys from South Korea, Japan, and the United States were to meet in Tokyo to discuss responses to the North's refusal to give up its nuclear programme.

The USS Michigan is built to carry submarine-launched ballistic missiles and Tomahawk cruise missiles.

U.S. President Donald Trump called for tougher new U.N. sanctions on Pyongyang on Monday, saying the North was a global threat and "a problem that we have to finally solve".

"The status quo in North Korea is also unacceptable," Trump told a meeting with the 15 U.N. Security Council ambassadors, including China and Russia, at the White House. "The council must be prepared to impose additional and stronger sanctions on North Korean nuclear and ballistic missile programs."

South Korean and U.S. officials have feared for some time that a sixth North Korean nuclear test could be imminent. Speculation has grown that such a test, or another long-range missile launch, could coincide with the 85th anniversary of the foundation of the North's Korean People's Army on Tuesday.

The official China Daily said on Tuesday it was time for Pyongyang and Washington to take a step back from harsh rhetoric and heed the voices of reason calling for a peaceful resolution.

"Judging from their recent words and deeds, policymakers in Pyongyang have seriously misread the U.N. sanctions, which are aimed at its nuclear/missile provocations, not its system or leadership," the newspaper said in an editorial.

"They are at once perilously overestimating their own strength and underestimating the hazards they are brewing for themselves," it said.

In a phone conversation with Trump on Monday, Chinese President Xi Jinping called for all sides to exercise restraint.

Two Japanese destroyers conducted exercises on Monday with a U.S. aircraft carrier strike group that is also headed for Korean waters, sent by Trump as a warning to the North.

Trump has also sought to pressure China to do more to rein in its nuclear-armed neighbour.

China, North Korea's sole major ally, has in turn been angered by Pyongyang's belligerence, as well as its nuclear and missile programmes.

Regardless, North Korea has carried out nuclear and missile tests in defiance of successive rounds of United Nations sanctions.

Angered by the approach of the USS Carl Vinson carrier strike group, which could arrive within days, North Korea said the deployment was "an extremely dangerous act by those who plan a nuclear war to invade".

"The United States should not run amok and should consider carefully any catastrophic consequence from its foolish military provocative act," Rodong Sinmun, the official newspaper of North Korea's ruling Workers' Party, said in a commentary.