"We are in a race between cooperation and catastrophe," said former Sen. Sam Nunn, who helped organize the exercise, dubbed Black Dawn. "To win this race, we have to achieve cooperation on a scale we've never seen or attempted before."

Nunn spoke to reporters Tuesday, a day after the closed-door war games attended by top officials including the European Union's security chief, Javier Solana, and his new counterterrorism czar, Gijs de Vries.

In first part of the scenario, European officials were asked how they would respond to intelligence that al-Qaida had obtained enough highly enriched uranium to build a nuclear bomb.

In the second, they were confronted with computer projections and video displays illustrating the impact of terrorists exploding the device at NATO's headquarters on the outskirts of Brussels, immediately killing 40,000 people, overwhelming hospitals with hundreds of thousands of injured, spreading panic through Europe and plunging the world economy into turmoil.

"Once you are in this phase, there are no good options," said Michele Flournoy, senior adviser at the Center for Strategic and International Studies, who helped prepare the exercise.

More than 50 people from 15 countries and a dozen international organizations attended the exercise, mostly EU ambassadors but also civilian and military officials from NATO, the International Atomic Energy Agency, Interpol and other bodies.

Nunn appealed for the Europeans to step up funding for increased protection at sites where weapons-grade uranium and plutonium are stored, particularly in former Soviet states.

He said preventing al-Qaida from getting its hands on such material was the best chance of stopping it from building a bomb.

"It's well within al-Qaida's operational capabilities to recruit the technical expertise needed to build a crude nuclear devise," he said. "The hard part is getting the nuclear material, but we do not make it nearly hard enough."

Nunn, a Democrat from Georgia and former chairman of the Senate Armed Services Committee, helped push through a $10 billion program in 1991 to destroy and safeguard weapons of mass destruction in Russia and other former Soviet republics. But he said at least 60 percent of sites still must be secured.

He said European leaders should make good on pledges made two years ago as part of a $20 billion commitment by the Group of Eight to provide more funding for that program over 10 years.

They should also push President Bush and Russian President Vladimir Putin to do more when the G-8 group of world leaders meets next month in Georgia, he said.

Solana and NATO Secretary General Jaap de Hoop Scheffer convened the exercise to show the extent of the danger.

"The threat of catastrophic terrorism is not confined to the United States or Russia or the Middle East," Solana said. "The new terrorist movements seem willing to use unlimited violence and cause massive casualties."

Nunn urged increased protection for weapons-grade uranium kept at research sites, which are often poorly guarded university facilities; accelerated destruction of tactical nuclear weapons by the United States and Russia; enhanced international intelligence sharing; and more help to find new jobs for poorly paid Russian nuclear scientists.