American and South Korean troops are holding a 10-day logistics drill in the city of in Pohang amid growing tensions on the Korean Peninsula.
The joint drill, dubbed Operation Pacific Reach (OPR), involves about 2,500 US soldiers and 1,200 South Korean soldiers, along with 50 vessels, and is due to last until April 21.
US and South Korean military officials said the joint drill is not part of the annual joint Foal Eagle exercise, and is aimed at enhancing logistics capabilities which can be applied not only for military purposes but also for humanitarian assistance.
A US official told Reuters over the weekend that the drill began as the US Navy strike group Carl Vinson cancelled a planned trip to Australia and was moving towards the western Pacific Ocean near the Korean peninsula as a show of force.
Pyongyang has slammed the move, saying the deployment proves the US aggression against Pyongyang has entered a “serious phase.”
"We never beg for peace but we will take the toughest counteraction against the provocateurs in order to defend ourselves by powerful force of arms and keep to the road chosen by ourselves," an unidentified spokesman for the North’s foreign ministry said.
On Tuesday, North Korean state media warned of a nuclear attack on the US at any sign of a US pre-emptive strike and said the country was prepared to respond to any aggression by the United States.
"Our revolutionary strong army is keenly watching every move by enemy elements with our nuclear sight focused on the US invasionary bases not only in South Korea and the Pacific operation theatre but also in the US mainland,"the official Rodong Sinmun newspaper said.
Tensions have escalated sharply on the Korean peninsula with talk of US military action gaining traction following its missile strikes last week against Syria.
North Korea accuses the United States of plotting with regional allies to overthrow its government. Pyongyang says it will not relinquish its nuclear deterrence unless the US ends its hostile policy toward North Korea and dissolves the US-led UN command in South Korea.