KANDAHAR, Afghanistan - Suicide bombers and gunmen stormed a police compound in southern Afghanistan Thursday, opening fire and setting off explosions in a coordinated attack that killed six Afghan security forces.

American Black Hawk helicopters and at least eight U.S. armored vehicles rushed to support dozens of Afghan troops battling the assailants at the three-building police complex in restive Kandahar province. One suicide bomber pretended to be an ambulance driver and detonated his explosives after wounded officers were loaded inside the marked rescue vehicle, said Kandahar Police Chief Khan Mohammad Mujahid.

Three bombers blew themselves up, said Zalmai Ayubi, a spokesman for the provincial governor. A fourth man exchanged fire with Afghan troops.

"I heard a blast and after that continuous fighting with rocket launchers," said Kandahar provincial policeman Ashrafullah Agha. After a third large explosion, Agha cut off the interview.

Attacks and fighting in Afghanistan have intensified recently, with insurgents coming from neighboring Pakistan as the annual spring fighting season gets under way.

In a separate incident, an Afghan border policeman who shot dead two U.S. Army soldiers earlier this week was killed on Thursday during a joint operation by Afghan and coalition troops, NATO said.

NATO said the operation took place in northern Faryab province and also led to the arrest of two insurgents.

"The successful operation resulted in the death of the individual responsible for the shooting. Two other suspected insurgents were detained for further questioning," NATO said in a statement.

On Monday, an Afghan border policeman shot and killed Sgt. Scott H. Burgess, 32, of Franklin, Texas, and Sgt. Michael S. Lammerts, 26, of Tonawanda, New York. Both had been training Afghan security forces. They had been assigned to the 1st Battalion, 84th Field Artillery Regiment, 170th Infantry Brigade Combat Team from Baumholder, Germany.

Monday's shooting took place inside a base in Faryab and the border policeman fled.

There have been numerous cases in the past of Afghan soldiers or police turning on their Western counterparts or of an insurgent disguising himself in uniform to infiltrate a compound and attack from inside.