"It is inexcusable for world leaders to not address the problem of securing nuclear material" which could fall into the hands of terrorist groups, former United States senator Sam Nunn told reporters at the end of a war games seminar in Brussels.

For now, Nunn said, extremists had trouble procuring nuclear material but that task was not being made hard enough.

"The most effective, least expensive way to prevent nuclear terrorism is to lock down and secure weapons and fissile materials in every country and in every facility that has them."

More than 50 officials and experts from 15 countries took part in the private seminar, organised by the Washington-based Center for Strategic and International Studies (CSIS), among them the European Union's foreign and security policy chief Javier Solana and the former United Nations chief weapons inspector in Iraq, Rolf Ekeus.

They acted out a doomsday scenario entitled "Black Dawn", which supposed that Al-Qaeda had secured some enriched uranium from a civil nuclear research centre in Europe and had managed to build a home-made nuclear bomb.

In the scenario, Al-Qaeda operatives explode their bomb at NATO headquarters, killing 40,000 people immediately and injuring another 300,000.

"It is well within (Al-Qaeda's) operational capabilities to recruit the technical expertise needed to build a crude nuclear device," the authors of the scenario claimed.

"A terrorist nuclear attack on US or European interests is consistent with Al-Qaeda's objectives and its profile."

The seminar paid particular attention to Russia, where large quantities of nuclear material are stocked under inadequate supervision.

"The scenario is fictitious but it is based on real facts," said CSIS expert Michele Flournoy, a former Pentagon expert. "There is ample evidence that they (Al-Qaeda) are going down this route."

She cited the discovery in 2001 at an Al-Qaeda base in Afghanistan of documents giving details of how to make a nuclear bomb.

Participants in the seminar, who included ambassadors and representatives of European institutions and NATO, recommended a series of priority actions.

These included the need to secure stocks of nuclear material, speed-up the dismantling of tactical nuclear weapons and to strengthen nuclear non-proliferation regimes.

"We are in a race between cooperation and catastrophe," Nunn said.

WAR.WIRE