SEOUL (REUTERS) - North Korea conducted a big live-fire exercise on Tuesday (April 25) to mark the founding of its military as a US submarine docked in South Korea in a show of force amid growing concern over the North's nuclear and missile programmes.

The port call by the USS Michigan came as a US aircraft carrier strike group steamed towards Korean waters and as top envoys for North Korea policy from South Korea, Japan and the United States met in Tokyo.

Fears have risen in recent weeks that North Korea would conduct another nuclear test or long-range missile launch in defiance of UN sanctions, perhaps on the Tuesday anniversary of the founding of its military.

But instead of a nuclear test or big missile launch, North Korea deployed a large number of long-range artillery units in the region of Wonsan on its east coast for a live-fire drill, South Korea's military said. North Korea has an air base in Wonsan and missiles have also been tested there.

"North Korea is conducting a large-scale firing drill in Wonsan areas this afternoon," the South's Office of Joint Chiefs of Staff said in a statement. The South Korean military was monitoring the situation and"firmly maintaining readiness", it said.

The South's Yonhap News Agency said earlier the exercise was possibly supervised by North Korean leader Kim Jong Un.

North Korea's state media was defiant in a commentary marking the 85th anniversary of the foundation of the Korean People's Army, saying its military was prepared "to bring to closure the history of US scheming and nuclear blackmail".

"There is no limit to the strike power of the People's Army armed with our style of cutting-edge military equipment including various precision and miniaturised nuclear weapons and submarine-launched ballistic missiles," the official Rodong Sinmun newspaper said in a front-page editorial.

North Korea's growing nuclear and missile threat is perhaps the most serious security challenge confronting US President Donald Trump.

Trump has vowed to prevent North Korea from being able to hit the United States with a nuclear missile and has said all options are on the table, including a military strike.

He sent the USS Carl Vinson aircraft carrier strike group for exercises in waters off the Korean peninsula as a warning to North Korea and a show of solidarity with US allies.

South Korea's navy said it was conducting a live-fire exercise with US destroyers on Tuesday in waters west of the Korean peninsula and would soon join the carrier strike group approaching the region.

China, North Korea's sole major ally which nevertheless objects to its weapons development, has repeatedly called for calm, and its envoy for Korean affairs, Wu Dawei, was in Tokyo on Tuesday.

"We hope that all parties, including Japan, can work with China to promote an early peaceful resolution of the issue, and play the role, put forth the effort, and assume the responsibility that they should," Chinese foreign ministry spokesman Geng Shuang told reporters in Beijing.

Japan's envoy on North Korea, Kenji Kanasugi, said after talks with his US and South Korean counterparts that they agreed China should take a concrete role to resolve the crisis and it could use an oil embargo as a tool to press the North.

"We believe China has a very, very important role to play,"said the U.S. envoy for North Korea policy, Joseph Yun.

South Korea's envoy, Kim Hong Kyun, said they had also discussed how to get Russia's help to press North Korea.

Japanese Prime Minister Shinzo Abe is expected to meet Russian President Vladimir Putin on April 27, the Kremlin said. It did not elaborate.

RARE SENATE BRIEFING

Matching the flurry of diplomatic and military activity in North Asia, the State Department in Washington said on Monday that US Secretary of State Rex Tillerson would chair a special ministerial meeting of the UN Security Council on North Korea on Friday.

Tillerson, along with Defence Secretary Jim Mattis, Director of National Intelligence Dan Coats and Joint Chiefs chairman General Joseph Dunford, would also hold a rare briefing for the entire US Senate on North Korea on Wednesday, Senate aides said.

On Monday, US President Donald Trump called for tougher new UN sanctions on Pyongyang, 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 UN 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 US officials have feared for some time that a sixth North Korean nuclear test or the latest in a string of missile launches could be imminent.

"SERIOUSLY MISREAD"

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 UN 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.

As the carrier group drills continued, the USS Michigan arrived in the South Korean port of Busan on Tuesday, the US Navy said. The nuclear-powered submarine is built to carry and launch ballistic missiles and Tomahawk cruise missiles.

As well as his military show of force, 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.

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