* Warnings mark biggest holiday on North Korean calendar
* North vows to never give up its nuclear arms
* Officials say South Korea remains vigilant for missile launch
* Kerry meets Japanese PM in Tokyo
North Korea made new threats of military action on Monday as the reclusive nation 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 Pyongyang tested a nuclear weapon in February.
"Our retaliatory action will start without any notice from now," North Korea's state news agency KCNA said on Monday, noting actions would "start immediately."
The statement was issued by the official North Korean news agency KCNA after signs that Pyongyang may be presenting a less warlike stance on the "Day of the Sun," the date the North's founder Kim Il-Sung was born.
Although many Pyongyang watchers had expected a big military parade to showcase North Korea's armed forces, the day was marked in Pyongyang with a festival of flowers named after Kim.
The United States has offered talks, but on the pre-condition that North Korea abandon its nuclear weapons ambitions. North Korea deems its nuclear arms a "treasured sword" and has vowed never to give them up.
Nevertheless, U.S. Secretary of State John Kerry, ending a trip to the region dominated by concern about North Korea, 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 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."
On Sunday evening, Kerry appeared to open the door to talking without requiring the North to take denuclearization steps in advance. Beijing, he said, could be an intermediary.
But on Monday White House spokesman Jay Carney said North Korea would have to "commit itself in a verifiable way to denuclearization" first, which has long been the U.S. position.
"If North Korea shows that it's serious about pursuing that path, then negotiations are the course through which that can be achieved," Carney told reporters.
Earlier, Kerry said he believed China, the North's sole economic and political benefactor, should put "some teeth" in efforts to persuade Pyongyang to alter its policies.
The Chinese Communist Party mouthpiece, the People's Daily, warned on Monday that tensions could get out of control.
"It does not matter if it is intentional or accidental, even the smallest thing could cause the situation to change rapidly and perhaps get totally out of control," the paper said.
South Korean and U.S. officials said last week North Korea appeared set to test-launch a medium-range missile as a show of strength linked to Monday's anniversary of the birth of North Korean state founder Kim Il-Sung.
North Korea has conducted three nuclear tests but it was not believed to be near weapons capability.
South Korea said it remained on guard against any missile launch and it regretted the North's rejection of an offer of talks made last week by President Park Geun-hye. It said the offer would remain on the table.
Missile launches and nuclear tests by North Korea are both banned under U.N. Security Council resolutions, which were expanded after its third nuclear test, in February.
President Barack Obama is under pressure from Republicans in Congress to respond forcefully should North Korea launch a missile.
"I maintain that the United States should treat any North Korean missile launch as a threat to our national security and our allies, and that we should shoot it down once it leaves North Korean airspace," said Senator John McCain, a Republican from Arizona who is influential on defense issues.
The aim of the North's aggressive acts, analysts say, is to bolster the leadership of Kim Jong-un, the 30-year-old grandson of the nation's founder, or to force the United States to hold talks with the North.
The North has also been angry about annual military exercises between U.S. and South Korean forces, describing them as a "hostile" act. The United States dispatched B52 and B2 stealth bombers from their bases to take part.
Yoji Koda, a former commander in chief of the Japanese naval fleet, told Reuters he believed North Korea's threats were part of a "calculated provocation", not a prelude to an attack.
He said indications the United States was softening a forceful stance to the North had given its leader Kim Jong-un the opportunity to tone things down.
"This has enabled North Korea to declare victory and sheathe its swords," he said.
Kim Il-Sung led his country from its founding in 1948 through the 1950-53 Korean War and until he died in 1994. His son, Kim Jong-il, then took over.
KCNA, the North's news agency, reported that people were flocking to a statue of Kim Il-Sung, saying, "My father, our great leader".
"This sincere expression comes from the bottom of their hearts," it said.
Kim Jong-un, the third Kim to rule in Pyongyang, attended a midnight celebration of his father's and grandfather's rule with top officials, including his kingmaker uncle Jang Song-thaek.
North Korean defectors said army units were expected to contribute for the celebration of Kim's birthday and in turn, the government, which has struggled to feed its people, had handed out extra rations of rice and corn.
"People are decorating streets for political events. It's never like war time," said Seo Jae-pyoung, a defector who lives in South Korea who spoke last week to an acquaintance in the North. "The government, which normally can't distribute rice, has already given about one week or two week's special rations."
North Korea has repeatedly stressed that it fears the United States wants to invade it and has manipulated the United Nations to weaken it. During the weekend, the North rejected the overture by new South Korean President Park as a "cunning" ploy.
"We will expand in quantity our nuclear weapons capability, which is the treasure of a unified Korea ... that we would never barter at any price," Kim Yong-nam, North Korea's titular head of state, told a gathering of officials and service personnel applauding the achievements of Kim Il-Sung.