Shooting of at least 10 police by colleague follows six American troops dying at hands of Afghan recruits a day earlier

An Afghan police officer has killed at least 10 of his fellow officers, a day after six US service members were shot dead by their Afghan partners.

Shakila Hakimi, a member of the Nimroz provincial council, said the policeman who opened fire on his colleagues at a checkpoint in Dilaram district on Saturday was believed to have had ties to militants. He was killed in an ensuing gun battle, she said in a telephone call from the provincial capital of Zaranj, along Afghanistan's western border with Iran.

"The checkpoint is in a remote area of a remote district," Hakimi said. "The telecommunications are poor and we are not able to get more details."

Hakimi said the provincial governor has sent a team to the scene to get more details about what happened.

On Friday six US soldiers were shot dead by Afghans, including police, in southern Helmand province – a grim reminder of the growing threat foreign forces face not just from the Taliban but also from their supposed allies. It was the bloodiest single day for foreign troops in the province since six British soldiers were killed by a roadside bomb in early March.

Three US Marine Corps special operations troops were shot dead in the early hours of Friday morning in Sangin district, a northern corner of the province that has seen heavy fighting. The killers were an Afghan police commander and some of his men, who had invited the US officers to join them for a meal and to discuss security, Afghan officials said.

Then in the evening an Afghan man shot dead three other foreign soldiers who worked on a joint base with him, the Nato-led coalition said. It does not reveal nationalities of soldiers killed during operations but Afghan officials said the men were from the US.

"The attack happened in police headquarters of Garmser," said Daoud Ahmadi, spokesman for the provincial governor, referring to a district about 40 miles (60km) south-west of the provincial capital. One other foreign soldier was also injured, he added.

The Taliban has claimed reponsibility for the attack. "Last night after prayer time around 9pm they were just coming out of the mosque and a policeman opened fire on the Americans outside the district police headquarters," said Taliban spokesman Qari Yousef Ahmadi.

The shooter was arrested, Nato said, but added that he was not wearing a uniform at the time of the attack, leaving open the possibility he was a civilian employee on the base. But Farid Farhang, spokesman for the provincial police chief, said the man was from a much-criticised auxiliary police unit usually trained by special forces. "All I know is that he was from the Afghan local police," Farhang said, adding that an investigation was under way. The attacks were the third and fourth times in less than a week that Afghans have turned on their mentors or colleagues.

On Tuesday two Afghan soldiers killed a US soldier and injured two others in eastern Paktia province, and on Thursday two other Afghan soldiers opened fire on a group outside another base in the east, although the only person killed was one of the shooters.

So far this year 37 soldiers and military contractors have been killed in 27 such attacks, far outpacing the toll in 2011. They have become such a commonplace threat that some foreign units are watched by armed "guardian angels" from their own ranks.

In a sign of growing concerns about the frequent shootings, President Hamid Karzai issued a rare condemnation of the deaths of foreign soldiers and ordered an investigation into shootings in Helmand and Paktia. "The enemy who does not want to see Afghanistan have a strong security force targets military trainers." Karzai said in a statement that also described the shooters as "terrorists in Afghan security uniform".

Nato commanders argue that the attackers account for only the tiniest portion of security forces, now numbering more than 300,000, and say many are driven by personal grudges rather than loyalty to the Taliban or other insurgent groups. But the shootings are disproportionately damaging to morale on the critical mission to train the Afghan police and army as foreign forces head home.

Two British soldiers were killed within the space of 48 hours in Helmand last week. Their deaths bring to 424 the number of UK soldiers killed in Afghanistan since 2001.