As North Koreans marked the 101st anniversary of the birthday of the country's late founder, Kim Il Sung, their government made a new threat of military action.
In an "ultimatum" to South Korea, the official news agency, KCNA, said Pyongyang's "retaliatory action will start without any notice from now."
The White House said Monday President Barack Obama and South Korean President Park Geun-hye will meet in Washington on May 7 to discuss economic and security issues, including "countering the North Korean threat.''
Concern about a missile test increased last week, after South Korea said it had seen the North moving mobile missile launchers on its east coast.
South Korean and U.S. forces remain on alert for any North Korean provocation. For weeks, Pyongyang has made repeated threats of possible nuclear attack -- including against the U.S. mainland.
The situation topped the agenda of Secretary of State John Kerry's trip to the region, which he wrapped up Monday in Japan.
Kerry said Washington is willing to negotiate with North Korea for a peaceful resolution of tensions on the Korean peninsula, if Pyongyang takes steps toward abandoning nuclear weapons. But he also repeated the U.S. commitment to the defense of both South Korea and Japan.
Kerry met in Tokyo with Japanese Prime Minister Shinzo Abe. He said the two had made important advances in cyber-security, climate change and on solidarity in the North Korean threat.
The top U.S. diplomat met with the Japanese leader after telling students at the Tokyo Institute of Technology that the burden is on Pyongyang to take meaningful steps to honor commitments it has already made. He added that the North must observe laws and the norms of international behavior.
Pyongyang has been angered by joint U.S.-South Korean military exercises that it sees as a prelude to an invasion of the North. Washington and Seoul have insisted the drills are defensive. North Korea also has reacted angrily to the latest U.N. sanctions imposed on it for carrying out a nuclear test in February. The United States and South Korea were among the main advocates of the sanctions.
Japan has deployed missiles around Tokyo to intercept any North Korean rockets launched toward its territory.