North Korea conducted a big live-fire exercise on Tuesday to mark the foundation 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 programmes.
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 Defence 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 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.
Japan's Chief Cabinet Secretary Yoshide Suga told media that China's nuclear envoy, Wu Dawei, would hold talks with Japanese Foreign Ministry officials on Tuesday. A ministry source said Wu was likely to meet his Japanese nuclear counterpart on 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.
Rare Senate briefing
Matching the flurry of diplomatic and military activity in Asia, the State Department in Washington said on Monday 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 Defense 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.
A North Korean Foreign Ministry spokesman said those meetings called by US officials clearly reflected the US pressure that could "ignite a full-out war" on the Korean peninsula.
"The reality of today again proves the decision to strengthen nuclear power in quality and quantity under the banner of pursuing economic development and nuclear power was the correct one," the unidentified spokesman said in a statement issued by the North's state media.
On Monday, Trump called for tougher UN sanctions on the North, saying it 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 programmes."
The official China Daily said on Tuesday it was time for Pyongyang and Washington to take a step back from harsh rhetoric and heed 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 sought to press 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 programs.