Eight Afghan policemen have been shot and killed in an attack on a checkpoint in the southern province of Helmand, officials say.
They say the attack was carried out by the Taliban near Lashkar Gah on Tuesday night. Three policemen were wounded and one is missing.
Some officials say that the attackers were helped by the missing policeman.
Lashkar Gah is one of seven initial areas where security has been handed from Nato to Afghan forces.
Foreign combat troops are due to withdraw from Afghanistan by 2014.
The BBC's Bilal Sarwary says that insurgents are trying to undermine the authority and lower the morale of Afghan security forces as the transition gets under way.
On Tuesday a suicide car bomber attacked a popular bakery in Lashkar Gah, killing at least five people.
High profile attacks
"The Taliban had help from a policeman inside the checkpoint," a senior policeman in the area told the BBC.
"Our information shows that the attackers were helped by this officer. He may have even have removed the [firing] pin from a heavy machine gun. He has fled the checkpoint with his gun.
"We are going to be looking for him. We have questioned two people who know him well.''
An Afghan intelligence official in the area told the BBC that the Taliban had managed to recruit the police officer some time ago.
"He did this for money because his record shows he was not a very religious person,'' the official said.
Correspondents say that in recent months Taliban militants have carried out several attacks, some of them high profile, by infiltrating the Western-trained Afghan security forces.
The bakery attack in Lashkar Gah took place close to police headquarters.
According to witnesses, police were buying bread when the attack took place. Two civilians were among the dead.
In a separate development, the New Zealand defence ministry has announced that a Special Forces soldier was killed in a gun battle with Taliban fighters during a special operation in the central province of Wardak.
He said that the soldier was part of a group of special forces troops supporting about 50 Afghan police trying to search for and arrest a rebel group suspected of planning an attack on Kabul.
Tags: Afghan police , Afghan policemen, Afghan security forces , Special Forces, Foreign Combat Troops, The Taliban, Taliban militants