By Li Hongmei

The Democratic People's Republic of Korea (DPRK) made new threats of military action on Monday as the country celebrated the anniversary of its founder's birth, stoking tension on the peninsula with a new "ultimatum" to South Korea in the stand-off over its nuclear program.

The latest statement from Pyongyang followed threats of nuclear attacks on the United States, South Korea and Japan, after new U.N. sanctions were imposed in response to the DPRK's latest nuclear test in February.

"Our retaliatory action will start without any notice from now," the DPRK's state news agency KCNA said on Monday, noting actions would "start immediately."

The statement was issued by KCNA after signs that Pyongyang may be presenting a less warlike stance on the "Day of the Sun," the date the DPRK's founder Kim Il Sung was born.

Although many Pyongyang watchers had expected a big military parade to showcase its armed forces, the day was marked in Pyongyang with a festival of flowers named after the founder Kim Il Sung.

U.S. President Barack Obama and South Korean President Park Geun-hye will meet May 7 at the White House to discuss economic and security issues, including "countering the threat" from the DPRK, the White House said on Monday.

U.S. Secretary of State John Kerry, ending a trip to the north-eastern Asia dominated by concern about the DPRK, stressed his interest in a diplomatic solution.

"The United States remains open to authentic and credible negotiations on denuclearization, but the burden is on Pyongyang," he said. "North Korea (the DPRK) must take meaningful steps to show that it will honor commitments it has already made, and it has to observe laws and the norms of international behavior."

But on Monday White House spokesman Jay Carney said Pyongyang would have to "commit itself in a verifiable way to denuclearization" first, which has long been the U.S. position.