"Combating nuclear terrorism is especially important today," US Undersecretary of State for Arms Control John Rood told a news conference.

"Regretably we continue to see indications in the United States from information we collect of the very terrorist groups we are most concerned about making concerted efforts to acquire nuclear capabilities with the express intent to use them against our peoples," he added.

Over 200 delegates from 56 nations, as well as the European Union and the International Atomic Energy Agency, are taking part in the three-day gathering which got underway in Madrid on Monday.

The meeting is the fourth of its kind since the Global Initiative to Combat Nuclear Terrorism was founded in 2006 by US President George W. Bush and Russian President Vladimir Putin to reinforce control of nuclear facilities and materials in order to prevent such groups from accessing them.

It was formed amid heightened global concern over nuclear programs in North Korea and Iran.

The closed-door gathering in Madrid is aimed at "developing and consolidating an international work plan of a technical nature to improve the fight against nuclear terrorism," Spain's foreign ministry said in a statement.

The world's five leading nuclear powers -- the United States, Russia, China, Britian and France -- form the core of the initiative which now includes 73 member states.

Ireland, Malta, Saudi Arabia and the United Arab Emirates are recent additions to initiative.

"Each year we see a considerable increase in membership of the initiative and that happens because there is an understanding that terrorism is a global, common threat that requires a global response," said Russian Deputy Foreign Minister Sergei Kislyak.

Previous meetings of the group have been held in Morocco, Turkey and Kazakhstan.

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