Media playback is unsupported on your device Media caption President Obama: ''There are still too many bad actors in search of these dangerous materials''

World leaders have called for closer co-operation to tackle the threat of nuclear terrorism at a summit on nuclear security in Seoul.

A communique at the end of the summit reiterated a joint call to secure "vulnerable nuclear material".

South Korean President Lee Myung-bak said nuclear terrorism remained a "grave threat", while US President Barack Obama said action was key.

The meeting was dominated by North Korea's plan to launch a rocket.

North Korea says the long-range rocket will carry a satellite when it goes up in April. The US says any launch would violate UN resolutions and constitute a missile test.

Iran's nuclear programme was also on the minds of the summit participants, with Mr Obama pledging to meet the leaders of Russia and China on the sidelines to work towards a resolution.

'Bad actors'

At the meeting, world leaders discussed measures to fight the threat of nuclear terrorism, including the protection of nuclear materials and facilities, as well as the prevention of trafficking of nuclear materials.

Analysis The communique describes nuclear terrorism as one of the most challenging threats to international security. But the responsibility to maintain security over nuclear materials lies firmly with states rather than international bodies. And any effort to try to establish or impose common international standards inevitably raises concerns in some quarters that the world's major powers are seeking to intrude into the nuclear affairs of other countries. That's why this communique reaffirms that measures to strengthen nuclear security will not hamper the rights of states to develop nuclear energy for peaceful purposes. The summit urges states to minimise the use of highly enriched uranium - one of the building blocks for a nuclear bomb. The summit highlights the threat from radioactive materials more generally. But again all the summit can do is urge states to take measures to secure these materials and work towards ratifying international conventions on nuclear security. It is hardly a resounding outcome from a gathering over-shadowed by the more immediate wrangling over North Korea's and Iran's nuclear activities.

A joint communique reaffirmed their commitment to nuclear disarmament, non-proliferation and peaceful uses of nuclear energy.

"Nuclear terrorism continues to be one of the most challenging threats to international security," it said.

"Defeating this threat requires strong national measures and international co-operation given its potential global, political, economic, social and psychological consequences."

But it omitted a reference made in a draft communique last Thursday on the need for "concrete steps" towards a world without nuclear weapons, AFP news agency reports.

There are currently no binding international agreements on how to protect nuclear material stored peacefully inside its home country, says the BBC's Lucy Williamson in Seoul. An amendment seeking to do that is still unratified after seven years.

Addressing the summit, Mr Obama warned there were still "too many bad actors'' who were threatening to stockpile and use ''dangerous'' nuclear material.

"It would not take much, just a handful or so of these materials, to kill hundreds of thousands of innocent people and that's not an exaggeration, that's the reality that we face," he said.

"The security of the world depends on the actions that we take."

Mr Hu called for "an international environment conducive to boosting nuclear security" to be created and Mr Lee called for concrete action to tackle a threat that posed "a grave challenge" to peace.

The summit was attended by almost 60 leaders from around the world.

Rocket launch

Meetings on Monday were overshadowed by North Korea's planned launch, scheduled to take place between 12 and 16 April.

Pyongyang says it is intended to mark the 100th anniversary of the birth of North Korea's founding leader Kim Il-sung.

Nuclear stockpiles in numbers Russia: 10,000

US: 8,500

France: 300

China: 240

UK: 225

Pakistan: 90-110

India: 80-100

Israel: 80

North Korea: fewer than 10 Source: Federation of American Scientists Nuclear weapons: Who has what?

On Tuesday, a North Korean foreign ministry spokesman said that the launch would go ahead as planned and criticised Mr Obama's stance as ''confrontational''.

North Korea "will never give up the launch of a satellite for peaceful purposes", the spokesman said in a statement in the official KCNA news agency.

A KCNA report also described the ''weather satellite'' Pyongyang planned to launch as useful for ''the study of weather forecast needed for agriculture and other economic fields''.

Japanese Prime Minister Yoshihiko Noda, speaking at the summit, called on Pyongyang to cancel the rocket launch, saying that it would violate UN Security Council resolutions.

"As such, the international community strongly urges North Korea to exercise restraint and cancel the launch," he said.

The resolutions were passed after a similar launch in April 2009. Japan is particularly concerned as that rocket was launched over the country three years ago.

The US and Chinese presidents met on Monday on the sidelines of the summit and agreed to co-ordinate their response to any "potential provocation" if Pyongyang went ahead with the launch.

South Korea and the US say North Korea risks further sanctions and isolation if it does not cancel its plans. Seoul has also warned it will shoot down the rocket if it strays over South Korean territory.