North Korea held a national memorial service on Sunday to mark the 100th day since the death of leader Kim Jong-Il, hailing the country's nuclear weapons programme as his outstanding feat.

Flags flew at half-mast nationwide, sirens and whistles sounded at noon (0300 GMT) and citizens observed three minutes' silence "in the humblest reverence", the official KCNA news agency said.

The whole country was "wrapped in an atmosphere of mourning", state media said.

"All institutions, industrial establishments, construction sites, rural and fishery farms, schools, streets, villages and families across the country were overcome with grief."

Kim Jong-Un, Kim's son and successor as leader, attended the service in the capital of Pyongyang, along with top party, state and military officials.

Premier Choe Yong-Rim, in a memorial address, praised the late leader for turning his country into "a world military power and a full-fledged nuclear weapons state in the most difficult period of the Korean revolution", KCNA said.

"This is the outstanding feat he performed for the eternal prosperity of the nation," Choe said, according to the agency.

Kim Jong-Un and top officials also visited the Kumsusan Palace of the Sun, where Kim Jong-Il's body is to be put on display, to observe a moment's silence and pay respects before a smiling portrait of the former longtime leader.

TV footage showed the plaza outside the palace crowded with people and troops.

Banners reading: "The great leader comrade Kim Jong-Il is with us forever," and: "Let's hold the great general Kim Jong-Il in high esteem for thousands of years to come," were hung at the plaza.

Kim Jong-Il died on December 17 of a heart attack at the age of 69. His untested son, aged in his late 20s, has been proclaimed the "great successor".

Tensions with the international community have risen since the North this month announced plans for a rocket launch to mark the 100th anniversary of the birth of founding president Kim Il-Sung, Kim Jong-Il's father.

Pyongyang says the launch, between April 12 and 16, will put a peaceful satellite into orbit.

The plan has been condemned by the United States, South Korea and other nations as a pretext for a long-range missile test banned by the United Nations.

The proposed launch is expected to dominate discussions on the sidelines of a two-day nuclear security summit opening Monday in Seoul, to be attended by US President Barack Obama and other world leaders.

The North held Kim's funeral on December 28 in snowbound Pyongyang, with Kim Jong-Un walking beside his father's hearse as tens of thousands of troops and civilians lined the streets.

Pyongyang says the leader died on a train en route to one of his "field guidance" trips, portraying him as a martyr to duty. He habitually wore a padded parka-style jacket during the bitter Korean winter.

"The jacket was as good as a uniform for the leader, who devoted his all to the country's development and people's wellbeing, true to the lifetime desire of President Kim Il-Sung," KCNA said, adding he was wearing it when he died.

"The image of smiling Kim Jong-Il, dressed in (a) jacket all his life, will be unforgettable forever for the Korean people," it said