Fifteen Taliban fighters who fought their way on to Helmand's main airbase destroyed six jets worth millions of pounds in an assault that was the start of a bloody weekend for Nato forces – in the last 48 hours six UK and US troops have died at the hands of their Afghan allies.

This year, more than 50 Nato soldiers have been shot by police or soldiers they mentor and fight alongside, nearly 15% of all foreign military deaths in Afghanistan. The number of attacks is rising steadily, and they are becoming a serious threat to the international mission in Afghanistan.

At Camp Bastion, the headquarters of both the US Marines and UK troops in Helmand, Friday night's attack caught commanders by surprise. The air base has long been considered virtually impregnable because of its isolated location in the middle of the desert and multiple defensive walls.

The attackers blew a hole in the outer perimeter of the base just after 10pm, then a 15-strong suicide squad flooded inside where they targeted the fleet of military aircraft.

They were dressed in US army uniforms and carried assault rifles, rocket-propelled grenade launchers and suicide vests, Nato said, describing the group as "well-equipped, trained and rehearsed". Although 14 were eventually killed and the injured survivor has been detained, they caused serious damage on the base.

The attackers killed two US soldiers and injured nine other people, completely destroyed six Harrier jump jets and "significantly damaged" two others. They also hit six of the aircraft hangars that protect planes and helicopters from the desert dust.

The sophisticated, destructive and high-profile assault served as a reminder of the resources and commitment of the Taliban at a time when Nato troops are streaming home for good, sometimes at a rate of hundreds a day.

Prince Harry, whom the Taliban had said they would target, is based at Camp Bastion for his deployment as an Apache helicopter pilot, but he was not harmed in the attack and former prime minister Sir John Major said bringing him home now would hand the insurgents a "propaganda triumph".

Hours after the firefight ended, and just a few dozen kilometres away, a "very reliable" member of the Afghan local police turned his gun on two British soldiers. He was not known to have Taliban connections, but had confessed suicidal thoughts to a colleague.

"We understand that he had mental health problems," said Daoud Ahmadi, spokesman for the provincial governor. "A day before the shooting this man was talking with another ALP guy and said 'I can't bear to continue with my life any more, I prefer death to life'," Ahmadi added.

Despite confusion about his motive, the killing appeared to be carefully planned. The man, Gul Agha, had bandaged his leg and pretended he needed medical help; he shot the two British soldiers with a pistol when they came to his aid, an Afghan security source told the Guardian.

He was shot dead in the ensuring firefight and when the corpses were examined, his leg was uninjured under the bandage, added the source, who asked not to be named.

Hours later, in the early hours of Sunday morning, four US soldiers were shot dead by an Afghan policeman at a checkpoint in the nearby province of Zabul, said Ghulam Jelani Farahi, the province's second most senior police officer.

"Last night about 2.30am, a man named Adel … opened fire on the American soldiers. He killed four, wounded three," Farahi said. "The initial investigation shows this man called the Americans and said, 'We are surrounded by Taliban, please come and help'. When they came to save the police from the Taliban, Adel opened fire with an AK-47."

Six other officers out of 15 on the checkpoint have also run away with their weapons, Farahi said.

Additional reporting by Mokhtar Amiri