US President Barack Obama warned Monday that nuclear terrorism remained one of the world's biggest threats, as he called for greater urgency in safeguarding the world's atomic stockpiles.

In a speech hours before the opening of a nuclear security summit in South Korea, Obama said major progress had been made over the past two years to make it harder for terrorists to get hold of material for atomic weapons.

"But we're under no illusions. We know that nuclear material -- enough for many weapons -- is still being stored without adequate protection," he said.

"We know that terrorists and criminal gangs are still trying to get their hands on it, as well as the radioactive material for a dirty bomb... the danger of nuclear terrorism remains one of the greatest threats to global security."

Obama urged the leaders or top officials from 53 nations who had gathered in Seoul for the two-day summit to "keep at it", and take concrete actions to secure nuclear materials.

Obama said he expected many nations to announce they had fulfilled pledges made at the inaugural nuclear summit in Washington two years ago, and for other countries to make new commitments on securing or removing material.

"This is the serious and sustained global effort we need. This is an example of more nations bearing the responsibility and the costs of meeting global challenges," he said.

The atomic programmes of North Korea and Iran were also set to be the focus of intense discussion on the sidelines of the summit, although they were not officially on the agenda of the two-day event.

In North Asia, tensions have escalated in recent weeks after North Korea announced it would launch a long-range rocket in April, which the United States believes is intended to test a missile capable of delivering an atomic warhead.

Obama again told North Korea on Monday to abandon its nuclear ambitions, warning its erratic and provocative behaviour would not be rewarded.

"By now it should be clear, your provocations and pursuit of nuclear weapons have not achieved the security you seek, they have undermined it," he said.

"And know this -- there will be no more rewards for provocations. Those days are over. This is the choice before you. This is the decision you must make."

The nuclear-armed North insists its rocket launch will merely put a peaceful satellite into orbit.

Obama and South Korean President Lee Myung-Bak had presented a united front against North Korea during a news conference on Sunday.

Obama is scheduled to meet separately with Chinese President Hu Jintao and Russian President Dmitry Medvedev on Monday ahead of the start of the summit.

Iran's nuclear programme, as well as North Korea's, was expected to be discussed during those meetings.

Obama warned Iran in his speech that time was running out to resolve the standoff over its nuclear programme through diplomacy.

"There is time to solve this diplomatically... but time is short. Iran must act with the seriousness and sense of urgency that this moment demands," he said.

Experts have acknowledged major progress on the fissile material front since the Washington summit two years ago.

They point to former Soviet republic Kazakhstan securing over 13 tonnes of highly enriched uranium (HEU) and plutonium since then, while Chile eliminated its entire HEU stockpile.

The United States and Russia also signed a protocol under which each will dispose of 34 tonnes of plutonium -- enough for 17,000 nuclear weapons.

Obama also said Monday he would raise the possibility of fresh nuclear arms cuts with Russia, and announced that all HEU had been removed from Ukraine.