U.S. Marines hunted Friday for gunmen who attacked their airport base in Kandahar during the first high-security flight of Al Qaeda and Taliban prisoners to a U.S. base in Cuba, where they will be questioned and possibly tried.

The attack triggered a brief but intense gunbattle, the first on the base since the Marines dug in a month ago. Officers claimed that the attack -- which erupted as a C-17 transport plane lumbered down the runway with 20 chained and hooded prisoners on board -- was unconnected to the transfer.

The Marines estimated that eight to 14 people armed with Kalashnikov assault rifles launched the probing attack from three different positions, getting to about 400 yards from the outer defensive perimeter of the sprawling air base.

Using ravines, ditches and abandoned mud houses as cover, the attackers sent aloft an illumination flare as the C-17 carrying the prisoners to Guantanamo Bay, Cuba, was taking off, then opened up with gunfire. No shots were aimed at the plane, Marine officers said. Intermittent shooting continued for about 40 minutes.

The Marines responded with M-16 assault rifles, grenade launchers and cannons, and sent up Cobra attack helicopters to seek the attackers. Eventually, a light armored vehicle went out to an abandoned mud house where some fire came from. No one was found.

"We used all the assets we have," Capt. Dan Greenwood, the operations officer who directed the Marine response, told reporters. "It was a probe, based on three fronts. They were testing our defenses, and they'll think twice about hitting us again."

There were no U.S. casualties. Patrols went out after sunrise, but found no bodies. A pair of rocket-propelled grenades were discovered in an abandoned mud house, perched on a moonscape of ravines and homes pounded to rubble after years of warfare.

The troops had been expanding control around the base in recent days to increase security for flights, but the firefight illustrated how the area around Kandahar, the birthplace of the overthrown Taliban regime, remains insecure.

"Quite simply, this is a very hard area to defend," said Marine 1st Lt. James Jarvis, spokesman for the Marines at the airport. "The airport takes up a lot of ground. It's a large piece of terrain to defend."

Jarvis and Greenwood said that they did not believe the attack was directly related to the prisoner flight.

"We kept the movement of the detainees a closely held secret, and we believe the two are completely isolated incidents," Jarvis said. "The longer we're here, the more enemy forces are going to take their chances."

The Marines were on a heightened state of alert for four hours after the attack, Jarvis said. Security procedures were being reviewed Friday.

Some 2,800 troops from the U.S.-led coalition are based at the airport. After Thursday night's flight to Cuba, 30 new prisoners were brought to Kandahar base's mud-walled detention center, ringed by high coils of razor wire, bringing the total to 361.

U.S. authorities plan to ship an undetermined number of prisoners from Usama bin Laden's Al Qaeda terror network and its Taliban allies to a more secure facility at Guantanamo Bay in Cuba.

Jarvis said that the first flight of 20 prisoners were of "various nationalities" but gave no other details of their identities or importance.

Authorities took no chances of a bloody uprising. Jarvis said that the prisoners were chained and hooded and brought 10 at a time under heavy guard by soldiers and police dogs to the aircraft.

Although the Taliban lost control of Kandahar and other major Afghan cities under the combined assault of U.S. airstrikes and offensives by Afghan fighters, security in the country is tentative at best. Many of the deposed militia's fighters have disappeared into Afghanistan's rugged terrain or have blended into civilian populations.

In the capital, Kabul, the advance guard of the German-Dutch peacekeeping mission arrived Friday to prepare the ground for the full force of 1,200 joining a British-led stabilization force charged with keeping peace.

British peacekeeping forces and newly deputized Afghan police formally launched joint patrols of the capital Thursday in an effort to restore security and civilians' trust. The new Afghan government has ordered men with guns off the city streets. Fewer have been seen.

Joseph Biden, the Delaware Democrat who heads the U.S. Senate Foreign Relations Committee, visited Kabul on Friday and said that the government was "exceeding expectations" in extending its authority, but predicted "you're going to see some glitches here and there."