North Korean state media has warned of a nuclear attack on the US at any sign of an American pre-emptive strike as a US Navy strike group led by a nuclear-powered aircraft steamed towards the western Pacific.
Tension has escalated sharply on the Korean Peninsula, with talk of military action by the US gaining traction following its strikes last week against Syria and amid concerns the reclusive North might soon conduct a sixth nuclear test.
North Korea's official Rodong Sinmun newspaper said on Tuesday the country was prepared to respond to any aggression by the US.
"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," it said.
South Korean Acting President Hwang Kyo-ahn warned of "greater provocations" by North Korea and ordered the military to intensify monitoring and to ensure close communication with the US.
"It is possible the North may wage greater provocations such as a nuclear test timed with various anniversaries including the Supreme People's Assembly," said Hwang, acting leader since former president Park Geun-hye was removed amid a graft scandal.
The North convened a Supreme People's Assembly session on Tuesday, one of its twice-yearly sessions in which major appointments are announced and national policy goals are formally approved.
But South Korean officials took pains to quell talk in social media of an impending security crisis or outbreak of war.
"We'd like to ask precaution so as not to get blinded by exaggerated assessment about the security situation on the Korean peninsula," defence ministry spokesman Moon Sang-kyun said.
Saturday is the 105th anniversary of the birth of Kim Il Sung, the country's founding father and grandfather of current ruler Kim Jong Un.
North Korea often also marks important anniversaries with tests of its nuclear or missile capabilities in breach of UN Security Council resolutions.
The North's foreign ministry, in a statement carried by its KCNA news agency, said the US navy strike group's approach showed America's "reckless moves for invading had reached 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 ministry spokesman said.
The US Navy strike group Carl Vinson was diverted from planned port calls to Australia and would move toward the western Pacific Ocean near the Korean peninsula as a show of force, a US official told Reuters at the weekend.
US officials said it would still take the strike group more than a week to arrive near the Korean Peninsula.