North Korea has carried out a big live-fire exercise to mark the foundation of its military, amid worries it will conduct another missile or nuclear test.
In a matching show of force, the United States is preparing for large scale maneouvres with South Korean armed forces.
The South Korean port call by the USS Michigan, which is designed to carry ballistic missiles and cruise missiles, came as a US aircraft carrier strike group steamed towards Korean waters in an effort to deter North Korea from a sixth nuclear test or more missile launches in defiance of UN sanctions.
Instead of a nuclear blast or a big missile test, North Korea marked Tuesday's 85th anniversary of the founding of its military by deploying a large number of long-range artillery units on its east coast for a live-fire drill, South Korea's military said.
South Korea's Office of Joint Chiefs of Staff said it was monitoring the situation and "firmly maintaining readiness".
South Korea's navy said it was conducting its own live-fire exercise with US destroyers in waters west of the Korean peninsula and would soon join the approaching US carrier group.
North Korea was defiant, 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.
He 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.
Mr Trump sent the USS Carl Vinson aircraft carrier strike group for exercises off the Korean peninsula as a warning to Pyongyang, but US officials said sanctions, not military strikes, were the preferred option.
On Monday, Mr Trump called North Korea a global threat and "a problem that we have to finally solve" and said the UN Security Council must be prepared to impose new sanctions.
US Secretary of State Rex Tillerson will chair a ministerial meeting of the Security Council on Friday to discuss tougher sanctions, which US officials said could include an oil embargo, banning North Korea's airline, intercepting cargo ships and punishing Chinese and other foreign banks doing business with Pyongyang.
On Wednesday, Mr Tillerson, Defence Secretary Jim Mattis, Director of National Intelligence Dan Coats and Joint Chiefs chairman General Joseph Dunford, are to hold a rare briefing on North Korea at the White House for the entire U.S. Senate.
China has 'very, very important role to play', US says
Japan's envoy on North Korea, Kenji Kanasugi, said he and his US and South Korean counterparts agreed in talks in Tokyo on Tuesday that China should take a concrete role to resolve the crisis and could use an oil embargo as a tool.
The US envoy for North Korea policy, Joseph Yun, said China had "a very, very important role to play" and South Korea's envoy, Kim Hong-kyun, said they had also discussed how to get Russia's help.
Japanese Prime Minister Shinzo Abe is expected to meet Russian President Vladimir Putin on 27 April, the Kremlin said. It did not elaborate.
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," Chinese foreign ministry spokesman Geng Shuang said.
North Korea's foreign ministry said the meetings called by US officials reflected US pressure that could "ignite a full-out war" and showed that Pyongyang's decision to become a nuclear power was correct.
The official China Daily newspaper said it was time to step back from harsh rhetoric.
"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."