Nuclear-armed North Korea on Tuesday marked a military anniversary with a massive conventional firing drill, Seoul said, as a US guided-missile submarine docked in South Korea amid tensions over Pyongyang's weapons ambitions.
Speculation had mounted that the North could carry out a sixth nuclear test or another missile launch to mark 85 years since the founding of its army.
But no such event -- which usually happens in the morning -- had taken place by mid-afternoon, and instead Seoul's defence ministry said Pyongyang was conducting a "massive fire drill" in the eastern port city of Wonsan.
The South's Yonhap news agency cited a government source as saying the exercise was the North's "largest ever" and presumed to have been overseen by leader Kim Jong-Un.
Washington has sent the aircraft carrier USS Carl Vinson to the Korean peninsula, where it is expected to arrive -- after a derision-provoking delay -- later this week.
Confusion had clouded the carrier's whereabouts earlier this month after US officials indicated it was sailing towards North Korea when in fact it was heading south.
The vessel will take part in joint naval drills with the South's forces to "demonstrate Seoul and Washington's strong determination to punish North Korean provocations", the South Korean Navy said in a statement.
They will take place in the East Sea, the South's name for the Sea of Japan, it said, and the two allies will also begin joint naval exercises in the West Sea -- what it calls the Yellow Sea -- on Tuesday "in relation to the current security situation".
The nuclear-powered US submarine USS Michigan also made a port call at Busan in the South on Tuesday in another show of force, while US navy destroyers carried out separate joint exercises with Japanese and South Korean vessels.
President Donald Trump has said the US was sending an "armada" to the Korean peninsula, including submarines.
According to the US Navy's Submarine Force Pacific website, the USS Michigan carries more than 150 Tomahawk cruise missiles.
They are capable of precision strikes against the North's nuclear facilities, but the South Korean Navy called the visit "routine" and said the submarine would not take part in any joint exercises.
- 'Brutal punishment' -
North Korea has ambitions to build a nuclear-tipped missile capable of reaching the US mainland. Tensions have soared in recent months as it carried out a string of missile tests that sparked tit-for-tat sabre-rattling between it and Washington.
Pyongyang's rhetoric always intensifies in the spring, when Seoul and Washington hold joint military drills which it sees as rehearsals for an invasion.
The North's Rodong Sinmun -- the official mouthpiece of the ruling Workers' Party of Korea -- warned Tuesday of dire consequences in the event of a US-led pre-emptive strike.
It promised "the most brutal punishment... in the sky and land as well as at sea and from underwater without any warning or prior notice".
North Korea launched two missile tests this month while Trump and his senior aides have warned that "all options are on the table" against Pyongyang, including military action.
Trump on Monday urged UN Security Council ambassadors to consider stronger sanctions against Pyongyang, and US senators will be briefed on North Korea at the White House on Wednesday.
Secretary of State Rex Tillerson is to chair a ministerial-level Security Council meeting on the North on Friday.
The North says it needs nuclear weapons to defend itself against invasion and a spokesman for its foreign ministry on Tuesday claimed the meetings -- coupled with Trump's fresh policy of "maximum pressure and engagement" on Pyongyang -- showed the threat it faced from the US.
"They are explicitly advocating pressure on us, which is as dangerous as lighting the fuse on an all-out war," he said in a statement carried by the state-run KCNA news agency.
Meanwhile, top nuclear envoys from Japan, South Korea and the United States met Tuesday in Tokyo and vowed "stern action" against any fresh North Korean provocations.
US special representative Joseph Yun said he discussed American policy to "enhance pressure" on Pyongyang "because we really do not believe that North Korea is ready to engage us towards denuclearisation".
The trio also agreed that China and Russia should play a bigger role in reining in the North, said Kim Hong-Kyun, South Korea's chief envoy on the issue.
"Cooperation with China and Russia is the most important for effective pressure on North Korea," he said.