Fortunately, it will be only a dress rehearsal for the real thing. Called "Topoff," the series of attacks by a fake foreign terrorist group is aimed at testing how top officials would respond if faced by real weapons of mass destruction.

As part of the staged attacks, actors will release mock biological agents, chemical weapons and radiation and hack into computers, said Ted Macklin, director of exercises and evaluations at the Justice Department's Office of State and Local Domestic Preparedness Support.

"This will give us the chance to assess in a context of a full-scale exercise how far we have come in the training process of our first responders and whether we need to make any midcourse corrections," he said in an interview.

"This exercise is very important to allow us to assess our national capacity and refine our approach to the threat of terrorism," he said, adding that the scale of the exercise - which will cost at least $3.4 million - was unprecedented in terms of the range of attacks.

Macklin refuses to provide specifics since he wants to avoid giving too much information to top officials, whom he wants to surprise with the carefully planned mayhem.

It has taken more than a year and the involvement of hundreds of state and local officials to plan the exercise that targets three venues: Denver, Portsmouth, New Hampshire, and an area outside Washington, D.C.

The only information to be disclosed so far is that the attacks will take place over a period of several days between May 7 and 26. No details will be released until then about the type of biological agents to be used, how the process will unfold and the specific sites in the cities chosen.

"The exercise is characterized as 'no notice.' We need an element of surprise," said Macklin. People can expect to see streets blocked off, emergency vehicles rushing to the scene and "victims" being taken to hospitals.

"People will see response vehicles, people may see people in protective gear and there will be visible activity. Where, when and what, we can't share with you," Macklin said. But he said his department was working hard to make sure people living in the areas affected knew that the events were simulated.

"People will understand that this is just an exercise, but they must also understand that if they see things that are actual emergencies they must tell officials," Justice Department spokesman Doug Johnson said.

Attorney General Janet Reno and Federal Emergency Management Director James Lee Witt are the first officials who have lined up to take part in the event.

Macklin said he hoped senior officials from the FBI, CIA, State Department, Pentagon and others would also participate. After the attacks, a report will be filed how they responded and recommend any improvements or corrections needed.

"Ultimately the end game in any exercise is that once the recom-mendations are validated we can incorporate them into training programmes to refine those," Macklin said. If a real attack takes place during the mock events, he said, enough resources have been set aside to deal with it.

The exercise stems from a provision inserted in a 1998 appropriations bill by Republican Senator Judd Gregg of New Hampshire calling for "practice operations" for a terrorist attack. His home state is one of those chosen for the mayhem.

Critics of the exercise say it overstates the real threats facing the United States and that such a broad range of attacks would be unlikely to happen together.

CIA director George Tenet told a Senate committee last month that a major threat to US security came from groups such as Saudi exile Osama bin Laden's that were trying to acquire biological and chemical weapons capabilities. He said, that over the next few years, US cities faced ballistic missile threats from a wide variety of sources.

(Agencies via Xinhua)