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

在首尔举行的全球核安全峰会上，各国领导人号召加强合作解决核恐怖主义。

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

Chinese President Hu Jintao urged the group to work together on the issue.

The meeting has so far been dominated by North Korea's plan to launch a rocket next month.

North Korea says the long-range rocket will carry a satellite. The US says any launch would violate UN resolutions and constitute（组成，构成） 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 are discussing 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.

There are currently no binding¹ international agreements on how to protect nuclear material stored peacefully inside its home country, says the BBC's correspondent in Seoul, Lucy Williamson. An amendment² 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 conducive³ 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, attending by almost 60 leaders from around the world, is due to issue a communique later in the day.