Insurgents opened fire on the police in the Arghandab district of Kandahar province, said deputy provincial police chief Amanullah Khan. Preliminary reports indicated that one of the policeman had links with the Taliban, he said.
Meanwhile, NATO's International Security Assistance Force said later Monday that two of its soldiers were killed in an explosion in southern Afghanistan. NATO said two other soldiers were wounded in the blast, which took place on Sunday.
NATO did not say which country the soldiers came from or where the blast occurred, but the British Broadcasting Corp. reported Monday that the troops were members of the Royal Air Force who were killed by a roadside bomb while on patrol in Kandahar.
Militants killed more than 925 Afghan police last year - more than 10 percent of the country's 8,000 insurgency-related deaths documented by the U.N.
Police make inviting targets for Taliban attacks. They have less training and less firepower than the Afghan army or NATO soldiers. They also tend to work in small teams in remote parts of the country where they can easily be overwhelmed by a small insurgent force.
Monday's ambush was the latest in a string of recent attacks on police in the south. Eight police were killed on Saturday - four while destroying opium poppies in Kandahar and four who were manning a checkpoint in Helmand. Seven police on the poppy-eradication force were killed April 7 in Kandahar.
U.S. officials say police are the focus of Taliban attacks because they are the weakest link in the country's security chain. Taliban militants often suffer devastating losses when they attack U.S. or NATO forces that have been stationed in the country since the 2001 U.S.-led invasion that drove the Taliban from power.
They have also largely abandoned ambush attempts against the increasingly capable Afghan army.