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.