US President Barack Obama told fellow leaders they have the world's safety in their hands, as a 53-nation summit on combating nuclear terrorism held its first full session on Tuesday.

"The security of the world depends on the actions that we take," he said in a speech at the start of the summit's second and final day AFP reported.

The meeting, which began late Monday, had been overshadowed by the nuclear ambitions of Iran and North Korea and the North's widely condemned plan for a long-range rocket launch.

On Tuesday the focus was on "non-state actors", terrorists said by the leaders to pose a grave threat.

Obama in 2009 declared his vision of a world without nuclear weapons and the following year hosted a Washington summit aimed at securing or destroying the world's stocks of plutonium and highly enriched uranium (HEU) by 2014.

"We are fulfilling the commitments we made in Washington," he said, adding security at nuclear facilities had been tightened and vulnerable material had been removed or destroyed.

"As a result, more of the world's nuclear material will never fall into the hands of terrorists who would gladly use it against us," Obama said.

"What's also undeniable is that the threat remains. There are still too many bad actors in search of these dangerous materials and these dangerous materials are still vulnerable in too many places.

"It would not take much -- just a handful of so of these materials -- to kill hundreds of thousands of innocent people."

But as a consequence of the Seoul summit, "more of our citizens will be safer from the danger of nuclear terrorism", Obama declared.

China's President Hu Jintao also noted good progress since 2010 but said the situation "remains grave".

His country would deepen cooperation with the UN nuclear watchdog the International Atomic Energy Agency and help countries that want to convert reactors from HEU fuel to low enriched uranium.

Summit host, South Korean President Lee Myung-Bak, said the proliferation of nuclear weapons and the threat of nuclear terrorism "pose a grave challenge" to peace.

"Terrorists know neither mercy nor compromise; they will not hesitate for a moment in taking innocent lives to obtain their goals," he told the summit.

The world still has some 1,600 tons of HEU and 500 tons of plutonium -- enough to make more than 100,000 nuclear weapons, Lee said.

"In this age there is no place that can be free from nuclear terrorism. We all share the same fate," he said, calling for firm commitments at the summit.

The summit was to issue a communique at 4.30 pm (0730 GMT) and Lee will then hold a press conference to wrap up the event, the biggest diplomatic gathering hosted by his country.

Leaders were also to tackle the threat posed by loosely-guarded radioactive material in hospitals and other sites, which could be combined with high explosives to make a "dirty bomb".

Late Monday France, Belgium, and the Netherlands -- three of the world's top suppliers of medical isotopes -- announced plans eventually to phase out the use of HEU in the production process, under a deal with the United States.

Obama started his South Korea trip with a visit Sunday to the tense and heavily fortified border with North Korea -- a symbolic expression of the US commitment to help defend its ally.

On Monday, he made an unusual, direct appeal to the North's new leaders, telling them to "have the courage to pursue peace".

There would be no more rewards for provocations, he said in reference to Pyongyang's scheduled rocket launch next month.

The North says it will launch a peaceful satellite. The United States and other nations consider it a disguised missile test, which would breach UN resolutions and a US-North Korea deal reached last month.

Hu, whose country is North Korea's sole major ally, reportedly expressed serious concern at Pyongyang's plan during a meeting Monday with Obama.

Neither North Korea or Iran were on the formal summit agenda but Seoul officials say the proceedings will send them a denuclearisation message.