KABUL - Four soldiers fighting with the NATO-led alliance were killed in an attack believed to involve members of the Afghan police in southern Afghanistan on Sunday, the coalition said.

The attack came a day after two British soldiers were shot dead by an Afghan policeman while returning from a patrol in southern Helmand province, one of the strongholds of the Taliban-led insurgency.

At least 51 foreign military personnel have been killed in "insider" attacks this year, attacks which have put a heavy strain on trust between the coalition and Afghanistan as they move towards handing security responsibility to Afghan forces by the end of 2014.

The rise in such attacks has led to the training of new recruits to the Afghan army and police being suspended.

With foreign combat troops withdrawing from the increasingly unpopular and expensive war, the enormous cultural divide that still separates Afghans and their allies after 11 years of conflict has become more of a concern than ever.

The NATO-led coalition and its Afghan counterparts have created a special Joint Casualties Assessment Team to investigate every attack, which number at least 37 this year.

In more than half of cases, attackers are either killed or escape and the motive never emerges, making it more difficult for the coalition to stem the surge.

Adding to the toll of coalition deaths caused by insider attacks over the weekend, two were killed and nine wounded in Friday's attack on Camp Bastion, one of the worst attacks on a NATO-operated base all year.

Six Harrier jets were destroyed and two were significantly damaged in the raid on Camp Bastion airfield, carried out by 15 insurgents split between three teams, a NATO statement said on Sunday.

Three refuelling stations were destroyed and six aircraft hangars were damaged.

All but one of the attackers were killed, with the remaining fighter taken into custody by coalition forces.

In a separate incident on Sunday, NATO-led forces arrested a Taliban fighter responsible for killing two U.S. troops when they were downed in their Kiowa helicopter in eastern Afghanistan, according to a separate statement by the coalition.