The nuclear-powered submarine USS Michigan approaches to join the U.S. aircraft carrier USS Carl Vinson in drills near the Korean Peninsula.

The nuclear-powered submarine USS Michigan approaches to join the U.S. aircraft carrier USS Carl Vinson in drills near the Korean Peninsula.

North Korea conducted a big live-fire exercise Tuesday to mark the founding of its military, media reported, as a US submarine docked in South Korea in a show of force amid growing concern over Pyongyang’s nuclear and missile programs.

The port call by the USS Michigan came as a US aircraft carrier strike group steamed toward Korean waters and as the 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 could 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.

South Korea’s Yonhap News Agency reported that the North appeared to have deployed a large number of long-range artillery units in the region of Wonsan on its east coast on Tuesday for a live-fire drill.

The report, citing an unidentified South Korean government source, said the exercise was possibly supervised by North Korean leader Kim Jong Un.

South Korea’s Defense Ministry said it could not immediately confirm the report.

“Our military is closely monitoring the North Korean military’s movement in Wonsan areas and we are firmly maintaining readiness,” South Korea’s Office of the Joint Chiefs of Staff said.

North Korea’s state media was defiant in a commentary marking the 85th anniversary of the founding 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 miniaturized 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.

Japan’s Chief Cabinet Secretary Yoshide Suga told media that China’s nuclear envoy, Wu Dawei, would hold talks with Japanese Foreign Ministry officials Tuesday. A ministry source said Wu was likely to meet his Japanese nuclear counterpart Wednesday.

Japan’s envoy on North Korea, Kenji Kanasugi, said after talks with his US and South Korean counterparts that they all 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 US envoy for North Korea policy, Joseph Yun.