At least four Canadian soldiers have been killed and at least eight others wounded in a so-called friendly fire incident near Kandahar in Afghanistan. The pilot of a U.S. F-16 apparently thought the Canadians were enemy forces and dropped at least one bomb on them.

Defense Secretary Donald Rumsfeld says it was a tragic accident. But speaking to U.S. military personnel in the midwestern state of Illinois Thursday, Mr. Rumsfeld says details of how it happened are under investigation and declines to comment.

"What we do know is some very fine coalition partners of ours from our neighbor to the north, Canada, were killed and they were killed by one or more bombs dropped from an F-16," he said.

U.S. defense officials believe the F-16 pilot apparently acted out of self-defense, thinking he was under fire from the ground and did not realize the Canadians were involved in a live-fire training exercise.