AVA- The insider attack took place in Takhar province, northeastern Afghanistan, on Thursday morning, provincial police spokesman Khalil Asir said.
The policeman, he said, had opened fire on his colleagues sleeping in a checkpoint in Khwaja Ghar district, and then set their bodies on fire.
The spokesman said the attacker was apparently linked to the Taliban as he took the dead fellows' weapons and fled to join the militants.
The attack came a few days after a similar incident, in which an Afghan policeman killed an American soldier in Logar province, east of Afghanistan, increasing the number of US troopers killed in insider attacks in Afghanistan this year to two.
Forty-seven insider attacks were carried out from December 2017 to May 2018 in various parts of the war-torn country, leading to the deaths and injuries of numerous Afghan police forces.
A Pentagon report released in June shows the casualties of the so-called "green-on-green" (Afghan forces attacking fellow Afghan officers) and "green-on-blue" (Afghan forces attacking US troopers) attacks have increased by 50 percent.
Overall, 3,555 US-led troops, including 2,414 American soldiers, have been killed since the US war on Afghanistan began in 2001 following the 9/11 terrorist attacks, according to iCasualties website.
The US invaded Afghanistan in 2001 with the declared aim of toppling the Taliban. The war removed the militant group from power but some 17 years on the Taliban are still active in two-thirds of the country and involved in widespread militancy, killing thousands of civilians as well as Afghan and US forces despite the presence of foreign troops.
Last year, the US added thousands of additional troops to Afghanistan. Washington claims the American troops are deployed to train Afghan forces and conduct counter-terrorist missions against militant groups.