U.S. President Donald Trump, who has urged China to do more to rein in its impoverished neighbour, said in a Tweet North Korea was "looking for trouble" and the United States would "solve the problem" with or without China's help.
Tension has escalated sharply on the Korean peninsula with talk of military action by the United States gaining traction following its strikes last week against Syria and amid concerns the reclusive North may soon conduct a sixth nuclear test.
North Korea's official Rodong Sinmun newspaper 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 U.S. invasionary bases not only in South Korea and the Pacific operation theatre but also in the U.S. 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 United States.
"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.
Trump said in a Tweet a trade deal between China and the United States would be "far better for them if they solved the North Korea problem".
"If China decides to help, that would be great," he said. "If not, we will solve the problem without them!"
Trump and his Chinese counterpart, Xi Jinping, met in Florida last week and Trump pressed Xi to do more to rein in North Korea.
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. It did not immediately release details.
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.
A military parade is expected in the North's capital, Pyongyang, to mark the day. North Korea often also marks important anniversaries with tests of its nuclear or missile capabilities in breach of U.N. Security Council resolutions.
Men and women in colourful outfits were singing and dancing on the streets of Pyongyang, illuminated by better lighting than that seen in previous years, apparently practising for the parade planned.
Syrian President Bashar al-Assad sent a message of congratulations to mark the event, lambasting "big powers" for their "expansionist" policy.
"The friendly two countries are celebrating this anniversary and, at the same time, conducting a war against big powers' wild ambition to subject all countries to their expansionist and dominationist policy and deprive them of their rights to self-determination," the North's KCNA news agency quoted the message as saying.
The North's foreign ministry, in a statement carried by KCNA, said the U.S. 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.
North Korea and the rich, democratic South are technically still at war because their 1950-53 conflict ended in a truce, not a peace treaty. The North regularly threatens to destroy the South and its main ally, the United States.
North Korea is emerging as one of the most pressing foreign policy problems facing the Trump administration.
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