The United States will launch what it described as its largest homeland security drill  in its history beginning on 12 May, the Washington Post reported on Monday.

Dubbed TOPOFF 2 (for top officials), the exercise will test the response to widely dispersed, almost synchronised attacks using weapons of mass destruction (WMDs), including radiological dispersal devices or "dirty bombs."

The drill, to take place in Seattle and Chicago, will cost an estimated $16 million and involve more than 100 federal, state, and local agencies, the American Red Cross and Canadian government agencies and organisations.

"We want to be able to test strategies, responses and protocols," said Homeland Security Secretary Tom Ridge in an interview with the Post.

"When you simulate people and technologies interacting, you learn a lot about your response capabilities."

Another Homeland Security official said the idea was to stimulate "lots of 'free play' where events are not scripted. We want to play out actions and see what goes on."

This will be Washington's second homeland security drill.

The first excercise took place in Denver three years ago and involved a staged germ warfare attack. At the time, news accounts described a chaotic response, with critical decisions taken late and different government agencies all competing to take the same reins.

Some 8,500 people are expected to take part in the mock attack, including officials in Washington, D.C., Maryland and Virginia. Planning for the drill began in June 2001.