WASHINGTON -- In the worst loss of life for the U.S. military in Afghanistan in almost a year, four Americans were killed yesterday when their Army helicopter crashed near the Bagram air base, defense officials said.

There were no indications that the crash of the UH-60 Black Hawk was caused by hostile fire, said Lt. Col. Martin Compton, a spokesman for U.S. Central Command, which is in charge of military operations in the region. Officials said that the cause of the crash was not clear, but that it appeared to be an accident.

The helicopter went down about 7:30 p.m. local time in an area known as the East Training Range, about 7 miles east of the air base that is the center of U.S. military operations in Afghanistan, Compton said.

Early today a powerful bomb destroyed a bridge outside the southern Afghan city of Kandahar, killing 18 people on a bus, a deputy police chief said.

No one immediately took responsibility for the explosion, but police blamed fugitive members of the Taliban and al Qaida. Police believed Afghan soldiers were the target of the explosion,