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UNITED NATIONS (AP) _ The United Nations and the New York City Fire Department have conducted their first joint exercise _ tackling a mock explosion and fire in the 39-story U.N. headquarters building, a U.N. spokesman said Monday.

In Sunday's drill, 26 vehicles and 150 firefighters responded to the hypothetical explosion on the 26th floor and a subsequent fire that included two possible casualties, spokesman Fred Eckhard said.

The U.N. headquarters building overlooking Manhattan's East River was reported to be a terrorist target following the 1993 bombing of the World Trade Center.

The New York landmark was evacuated after the Sept. 11 attacks, and again the following day, and mail delivery was halted for several weeks because of the anthrax threat.

The United Nations has been condemned by Osama bin Laden, the suspected mastermind of the Sept. 11 suicide hijackings. In a videotaped statement in November, he accused the world body of siding with the United States.

To make the simulation as real as possible, smoke machines pumped non-lethal smoke into the 26th and 27th floors, and once those floors had filled up sufficiently with the simulated smoke, the fire alarm was sounded, Eckhard said.

Water was pumped into the 26th floor and it was later mopped up, he said.

``The main thing was to test the communications between our security personnel and the fire department,'' he said.

Afterward, U.N. security officials ``agreed that the exercise had gone well,'' Eckhard said.

But there was one casualty: the U.N.'s fire pump couldn't handle the 500 pounds of water pressure being pumped by the New York Fire Department and developed some leaks. It is being repaired, said U.N. spokesman Farhan Haq.