SEOUL, South Korea — North Korea celebrated the 101st anniversary of its founder's birth with flowers on Monday, although there was no sign of tension easing as South Korea warned that the North's survival could be in question without change and development.

The North has threatened for weeks to attack the United States, South Korea and Japan after new U.N. sanctions were imposed in response to its latest nuclear arms test in February.

The United States has offered talks, but on the pre-condition that North Korea abandons its nuclear weapons ambitions. North Korea deems its nuclear arms a "treasured sword" and has vowed never to give them up.

South Korea's Defense Ministry said it remained on guard against any missile launch to coincide with the Day of the Sun, the date state founder Kim Il Sung was born, although such fears appeared unjustified as the day passed.

"The military is not easing up on its vigilance on the activities of the North's military with the view that they can conduct a provocation at any time," a ministry spokesman said.

In the North's capital, Pyongyang, the anniversary was marked with a festival of flowers named after Kim, the grandfather of the North's unpredictable new leader, Kim Jong Un.

In contrast to tirades against its enemies, including threats of nuclear war, North Korean state media made no mention of preparations for conflict on Monday.

KCNA, the North's news agency, reported that people were flocking to a statue of Kim Il Sung, uttering "My father, our great leader."

"This sincere expression comes from the bottom of their hearts," it said.

Kim Il Sung was born in 1912 and 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.

Expectations had been that Kim Il Sung's birthday would be marked with a mass parade to showcase the North's military might, but there was no immediate word if that was happening. In 2012, following the death of his father, Kim Jong Un made a public speech, the first in living memory for a North Korean leader.

The South Korean Unification Ministry, which oversees relations with the North, said it was "regrettable" that the North rejected 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, that were expanded after its third nuclear test, in February.

The aim of the North's aggressive acts, analysts say, is to bolster the leadership of Kim Jong-un, 30, or to force the United States to hold talks with the North.

South Korean Foreign Minister Yun Byung-se said the North Korean regime had to be persuaded to change for any hope of economic development.

"Without change, the mere survival of the regime may be in question," Yun told a conference of journalists.

"DECORATING STREETS"

Kim Jong Un, the third Kim to rule in Pyongyang, attended a midnight celebration of his father and grandfather's rule with top officials, including his kingmaker uncle Jang Song Thaek and the country's top generals.

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 recently 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."

In Tokyo, Secretary of State John Kerry said Washington was "open to authentic and credible negotiations on denuclearization."

Kerry's trip to South Korea, China and Japan was aimed at reassuring its allies and putting pressure on Beijing to act decisively to implement the U.N. sanctions.

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.

If matters did go out of control, it said, "no party will be able to stand on the side".

North Korea has repeatedly stressed that it fears the United States wants to invade it and has manipulated the United Nations to weaken it. At 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.