Two Afghan police officers who had recently rejoined the force after defecting to the Taliban, shot dead seven of their sleeping colleagues, a police chief said.
The killings came during a particularly bloody 24 hours for Afghan forces, with another 16 soldiers, police and bodyguards killed in different attacks, underscoring concern about government forces as foreign troops prepare to leave.
Kandahar Afghan National Police chief Abdul Raziq said the two police officers had defected to the Taliban months ago but returned several days ago asking to rejoin.
They were accepted back.
"As soon as the policemen fell asleep, the pair grabbed weapons and opened fire, killing all seven," Raziq said.
The attack occurred in the early hours in the Arghistan district of the southern province of Kandahar.
A hunt was on to find the pair, Raziq said, adding that he suspected they have again joined the Taliban.
The Taliban, fighting to expel Western forces and establish Islamist rule in Afghanistan, claimed responsibility in a text message from spokesman Qari Yousuf Ahmadi.
"An infiltrated Taliban killed 12 policemen including a commander. He brought a police vehicle, weapons and ammunition to Taliban," Ahmadi said.
The militants routinely exaggerate casualty numbers and often provide misleading information about their attacks.
They have stepped up attacks in recent weeks after a traditional winter lull in fighting.
Nine government soldiers were killed during separate attacks around the country on Tuesday, including five in the remote northeast province of Badakhshan and two in Kandahar.
In other attacks, at least three policemen were killed by a roadside bomb, also in Kandahar, and another bomb killed four bodyguards of the head of a private communication company in the central province of Parwan.
Concern is mounting over how the 352,000 members of Afghanistan's security forces will cope after most foreign NATO-led combat troops leave by the end of next year.