US security officials began a five-day anti-terrorism exercise in the eastern states of Connecticut and New Jersey on Monday, to train first responders with fake biological and chemical terror attacks.

The drill, named "TOPOFF 3," started with police probing a fakecar accident on a college campus and health officials getting prepared to deal with a mock biological attack in New Jersey, and included a fake chemical weapons attack in Connecticut on Monday afternoon.

According to the plan of the exercise, terrorists who have plotted to attack New York and Boston and suspected their plans have been compromised launch a premature attack by dispersing a biological agent from a car in New Jersey and a chemical attack inthe town of New London, Connecticut.

Although no real weapons were to be used, the participants would respond as if the exercise was a real attack with real weapons and real consequences, a Homeland Security Department statement said.

More than 10,000 people were expected to participate in the 16 million US dollar event. Canada and Britain would conduct related exercises simultaneously in their own countries, and 13 countries would have observers to the drill.

The drills were monitored by Homeland Security officials from acommand center in Washington and regional centers in New Jersey and Connecticut.

"TOPOFF 3" is the third exercise involving simulated terrorist chemical, nuclear or biological attacks. The first was held in May2000 and the second in May 2003.