A US submarine has arrived in South Korea, amid worries of another North Korean missile or nuclear test.
The missile-armed USS Michigan is set to join an incoming group of warships led by aircraft carrier Carl Vinson.
North Korea is celebrating its army's 85th founding anniversary on Tuesday. It marked the event with a large-scale firing drill, South Korea said.
Tensions have risen in the area in recent weeks, with the US and North Korea exchanging heated rhetoric.
Experts fear Pyongyang could be planning more tests - it has marked some key anniversaries in the past with nuclear tests or missile launches.
However, South Korea's defence ministry said "no unusual development had been detected".
Instead, the North conducted a large live-fire drill around the city of Wonsan, South Korea said.
"Our military is closely monitoring the North Korean military's movement," the Office of the Joint Chiefs of Staff said.
North Korea conducted a failed ballistic missile test on 16 April, prompting US Vice-President Mike Pence to warn it not to "test" President Donald Trump.
In an unusual move, the entire US Senate has been asked to attend a briefing on North Korea on Wednesday at the White House.
The USS Michigan docked at South Korea's Busan port on Tuesday, in what it called a routine visit. It is a nuclear-powered submarine carrying 154 Tomahawk cruise missiles and 60 special operations troops and mini-subs, reported the South Korean newspaper Chosun Ilbo.
Analysis: Stephen Evans, BBC News, Seoul
The North Korean crisis is moving on many fronts.
The US submarine docking in South Korea is a routine event - but in this time of heightened tension it has extra significance
Pyongyang remains defiant, including of China, warning that if it helped the US, the results would be "catastrophic".
Envoys from South Korea, the US and Japan are discussing North Korea in Tokyo.
What this all adds up to depends on whether President Trump has rejected the advice given to his predecessors that attacking North Korea could provoke an attack on Seoul, with one expert saying there could be tens of thousands of deaths on the first day.
It may be that President Trump has decided that the cost of North Korea eventually getting nuclear weapons that could strike the US means that the risk of war has to be taken. We simply do not know.
The submarine is expected to take part in military exercises with the Carl Vinson warship group, which the US said it was dispatching to North Korea earlier this month to "maintain readiness" in the region.
At the time, Mr Trump said that he was sending an "armada" to the region and that the US had submarines which were "very powerful, far more powerful than the aircraft carrier".
Pyongyang reacted angrily to the aircraft carrier deployment, threatening to sink it and launch a "super-mighty pre-emptive strike" against what it called US aggression.
However, the US warships caused some confusion and attracted mockery when it emerged that they actually sailed in the opposite direction, away from North Korea, after the announcement. However, US Navy officials said they are now proceeding to the region as ordered.
China is North Korea's only ally and main trading partner - and the US has been urging Beijing to help put pressure on Pyongyang.
Chinese President Xi Jinping spoke to Mr Trump on Monday, urging all sides to "maintain restraint and avoid actions that would increase tensions".
'Swords at the ready': The escalating rhetoric of a crisis
Pyongyang has long been known for its overblown statements but, increasingly, the US is matching its tone. Here are some recent examples:
- "In the case of our super-mighty pre-emptive strike being launched, it will completely and immediately wipe out not only US imperialists' invasion forces in South Korea and its surrounding areas but the US mainland and reduce them to ashes" - Rodong Sinmun, 20 April
- "The shield stands guard and the sword stands ready" - US Vice-president Mike Pence, 19 April
- "Now that the US has pulled out its sword to kill us, we will also pull out our grand sword of justice and fight till the end, and we will kill the U.S. imperialists with our strong and revolutionary power" - North Korean channel KRT, 25 April