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. READ MORE
