The international coalition in Afghanistan says four service members were killed Sunday in an “insider” attack in the southern part of the country.

Local authorities in Zabul province say the four were Americans, and that the killer was an Afghan police officer. Coalition officials have not identified the service personnel.

Sunday's attack comes one day after two British solders were gunned down by an Afghan policeman in southern Helmand province.

The deadly attacks are the latest in a series of so-called “insider” attacks by Afghan forces against their international counterparts, threatening the military partnership between NATO and Kabul.

In another development, coalition officials said Sunday the insurgents who attacked a British military base in Afghanistan Friday, killing two U.S. Marines, were dressed in U.S. Army uniforms and were armed with automatic rifles, rocket propelled grenade launchers and suicide vests.

The coalition says the 15 militants were organized into three teams and “executed a well-coordinated attack” against the airfield on Camp Bastion in Helmand province.

The alliance say the attackers destroyed six Harrier jets and “significantly damaged” two more. The militants also destroyed three coalition refueling stations and damaged six aircraft hangars.

International troops say they killed 14 of the insurgents and wounded one, who is in custody.

The Taliban has claimed responsibility for the attack.