North Korea celebrated the birthday of late founding father Kim Il Sung on Wednesday while the isolated regime prepared to restart its nuclear reactor by expelling U.N. inspectors.

North Korea, which claims it sent a satellite into space April 5, reacted angrily Tuesday to the U.N. Security Council's condemnation of its launch. Diplomats called it a violation of resolutions that bar Pyongyang from ballistic missile-related activity, including firing long-range rockets.

The North vowed to boycott ongoing international talks on nuclear disarmament and restart its atomic program. On Wednesday, inspectors from the International Atomic Energy Agency were expelled from the country's Yongbyon nuclear site, a diplomat close to agency said.

The diplomat, who asked for anonymity because he was not authorized to release the information, says the IAEA experts removed all agency seals from the nuclear facilities as well as surveillance cameras before leaving for the capital.

Japan joined Russia and U.S. in urging North Korea to return to disarmament talks involving the four nations, South Korea and China that yielded a 2007 pact calling for Pyongyang to dismantle its nuclear program in exchange for aid.

"Based on close cooperation with all countries involved, starting with the U.S., we want to demonstrate progress in the six-party talks," Japan's Chief Cabinet Secretary Takeo Kawamura said Wednesday in Tokyo.

Russia's chief nuclear envoy, Deputy Foreign Minister Alexei Borodavkin, said Wednesday that the "most important task" now is to restore the nuclear talks — not trying to impose heavier sanctions on Pyongyang for the rocket launch, according to the ITAR-Tass news agency.

In Seoul, South Korean President Lee Myung-bak called for firmer defense readiness amid "ever-growing tension and uncertainty" on the divided peninsula, according to the test of a speech to mark the anniversary of the founding of the country's marine corps.

There was no official North Korean response to the international criticism Wednesday, which was celebrated nationwide as the "Day of the Sun" — a holiday honoring Kim Il Sung's birth 97 years ago.

North Korea has said it wants to build up the country's space program by the 2012 centenary of Kim's birth. The U.S., Japan and other nations denounced the launch as a furtive test of the regime's long-range missile technology.

The launch also provided a dramatic backdrop for the start of son Kim Jong Il's third term as leader months after he reportedly suffered a stroke.

The two Kims are the focus of an intense cult of personality in the nation of 24 million people. Their portraits hang in nearly every room, and many North Koreans wear small red lapel pins bearing the elder Kim's picture. The Kims' birthdays are treated as important national holidays.

Kim Il Sung ruled North Korea for more than four decades until his death from heart failure in 1994. Known as the "Great Leader," he remains the country's "eternal president" even while his son now leads.

On Wednesday, North Koreans laid bouquets and made deep bows before Kim Il Sung's statues across the country, state TV showed. Senior Workers' Party members and military officials also visited the Kumsusan Memorial Palace where Kim's embalmed body lies in state, the official Korean Central News Agency said.

"Unending adoration toward President Kim Il Sung is overflowing wherever (you) go in Pyongyang," KCNA said.

For weeks, North Korea has been leading up to the "Day of the Sun" with film and music festivals, athletic competitions, art exhibitions and a flower show featuring the late founder's namesake "kimilsungia" orchid.

In a highly unusual move, two Christian bands from the U.S., the Nashville, Tennessee-based classical-fusion Annie Moses Band and Grammy Award-winning Casting Crowns, were among musicians performing Wednesday at an international festival showcasing dancers, acrobats and musicians.

"We are there to demonstrate respect for the people and continue to establish relationships," Mark Hall, lead singer for Casting Crowns, said in a statement last week, ahead of a trip to the communist nation long criticized for not allowing religious freedom.

It is very rare for an American band to play in North Korea, one of the world's most isolated nations that puts heavy restrictions on Western pop culture. But the New York Philharmonic performed in Pyongyang last year — a short-lived sign of goodwill between the two countries.

In a move bound to further infuriate Pyongyang, South Korea has decided to join a U.S.-led program aimed at halting the spread of weapons of mass destruction, the presidential office confirmed Tuesday. The program mainly targets rogue states like North Korea.

Last month, North Korea warned Seoul it would consider its participation in the program "a declaration of war."