A lone gunman opened fire on U.S. troops searching caves in southeastern Afghanistan, a U.S. military spokesman said Saturday. No one was hit and the gunman was not captured.

The U.S. patrol was fired upon around noon Friday by an assailant hiding in the Adi Ghar mountain, the site of a firefight Monday between a group using the mountain caves as a supply depot and U.S. and coalition forces.

"After the smoke cleared, they went in and cleared the cave and were not able to find any evidence that the man was actually in there," military spokesman Col. Roger King said at Bagram Air Base. "It's a possibility that he fired, went behind a rock and they thought he went into a cave, and instead he went somewhere else."

Eighteen rebels were killed in Monday's battle. Since then, troops have inspected 31 of the caves looking for more enemies. At least a hundred caves, some small and others more complex, stud the mountainside.

"There's other evidence that's being found that people are still in the area," King said, without elaborating. "I don't want to go into specifics because they might stop doing what they're doing."

King also condemned as cowardly a deadly bus bombing Friday in Kandahar that killed at least 15 people. No one has taken responsibility for the explosion, although police blamed Al Qaeda and Taliban fugitives.

"If it is indeed an attack by Al Qaeda and Taliban, you'll notice who they attack -- defenseless women and children. They didn't come against their armed opponent because they know what happens when they come against their armed opponent," King said.

The attack was near the Kandahar Air Field, the largest U.S. presence in southern Afghanistan. King said it underscored that the U.S. mission to hunt down enemy forces in the country remains unfinished.

"Our operations aren't over. The war against terrorism in Afghanistan continues to be fought and it continues to be fought in a variety of places," he said.

The bodies of four U.S. servicemen who died when their Black Hawk helicopter crashed Thursday during a nighttime training mission have been taken back to the United States, King said. An investigation of the crash is ongoing.