Two rocket-propelled grenades were fired at an airfield in southern Afghanistan used by U.S. special forces troops, a military spokesman said Monday. No injuries were reported.

Col. Roger King said it was not known who fired the grenades late Sunday, which exploded within the grounds of the airfield near the southern city of Kandahar. He declined to say how close the explosions were to U.S. forces, though he said no equipment was damaged.

U.S. forces did not return fire because they could not determine which direction the grenades came from, King said. Special forces troops were searching for the source of the attack.

"It was enough of a concern that we wanted to go out and find out who was shooting at us," King told reporters at Bagram air base near Kabul, the capital, which is the headquarters of U.S. military operations in Afghanistan. "But it's not like somebody penetrated our perimeter."

He said the rocket-propelled grenades have a range of about 300 yards.

King also said there had been "sporadic gunfire" near the base during the weekend. He did not elaborate, and it was not clear if the firing was directed at the base. Many Afghans keep weapons, mostly automatic rifles, in their homes for security reasons.

The base is about 13 miles outside Kandahar, the heartland of the former Taliban regime, about 370 miles southwest of Kabul. About 5,000 U.S., Canadian and other allied troops are stationed in and around the Kandahar air base.

U.S. troops and their allies have repeatedly come under rocket fire in Afghanistan in recent weeks but have suffered no casualties.

Sur Gul, the security chief in the eastern town of Khost, said three rockets were fired in the direction of the Khost airport on Saturday but landed about a mile short.

U.S. and British troops have been searching the area around Khost and other provinces near the Pakistani border for al-Qaida and Taliban fighters. Few fighters have been found.

U.S. troops in Khost also came under rocket fire on June 24 and June 25 but suffered no causalities. They dispatched patrols to search for the source of the firing but the patrol's results weren't immediately known.

It is not known if al-Qaida, the Taliban or local warlords have been firing the rockets.

King said U.S. special forces patrols during the weekend found two large weapons caches in caves in southeastern Afghanistan, the latest in a series of weapons stockpiles uncovered by U.S. and British troops in recent weeks.

About a week ago, villagers directed American forces to three large weapons caches near their homes in what a U.S. official said was a sign of civilians' increased desire to rid southeast Afghanistan of hidden arms.