Afghan residents walk as smoke rises from the direction of a suicide bomb attack that targeted foreign military forces in Lashkargah city, capital of Helmand province, Nov. 15, 2015. NOOR MOHAMMAD/AFP/Getty Images

Taliban insurgents in captured military Humvee vehicles launched suicide attacks in the southern Afghan province of Helmand on Saturday, killing several members of the security forces in the district center of Sangin, a senior official said.

The incident comes amid bitter fighting in Helmand, a traditional Taliban heartland where insurgents have overrun many areas, leaving government forces in some district centers including Sangin and Marjah barely clinging on.

Provincial police chief Abdul Rahman Sarjang said that after heavy fighting Friday during which the Taliban lost around 40 fighters, suicide bombers in two captured Afghan army Humvees targeted the police and governor's headquarters.

"The first Humvee was ordered by the police to stop, but when he ignored warnings, the police fired a rocket-propelled grenade," Sarjang said.

The second bomber detonated his vehicle near a checkpoint guarding the two headquarters. Four policemen were killed and seven wounded, he said, while an army spokesman said one soldier was killed and another was wounded.

In Kabul, Gen. John Campbell, the departing commander of international troops in Afghanistan, confirmed the incident but said no American troops were involved.

"I think right now the Afghan forces have a plan to go after that," he said.

The Taliban have captured large amounts of weapons and equipment from Afghan government forces, including Humvees and other vehicles, as the insurgency has spread over the past year.

Campbell recently ordered around 500 special forces personnel into Helmand to help bolster struggling Afghan security forces, but their role is to act as trainers and advisers, and to get involved in combat only if they come under attack.

Last month in Helmand, an American special forces soldier was killed during a firefight near the district center of Marjah while he was on an operation with Afghan commandos.

U.S. and British soldiers and marines suffered hundreds of casualties in Helmand where they fought for years against the Taliban for control of the province, one of the world's biggest opium-producing areas.