A large explosion hit just outside the police headquarters in a southern Afghanistan city on Sunday, killing at least six police officers, officials said.

The blast was detonated in a parking lot outside the police building in Kandahar, said Saisal Ahmad, a spokesman for the provincial government. Six officers were killed and another 16 people were wounded, including 10 civilians, the provincial government said in a statement.

An Afghan security man carries a machine gun at the scene of a suicide attack in Kandahar on Sunday, Feb. 5, 2012. (Allauddin Khan/Associated Press)

No one immediately claimed responsibility for the blast, which shattered windows in nearby buildings.

Elsewhere, Afghan police said an American soldier shot and killed an Afghan guard at a U.S. base in the north, apparently because the American thought the guard was about to attack him.

There have been a growing number of attacks by Afghan soldiers against international forces in Afghanistan in recent years, some the result of arguments and others by insurgent infiltrators. Last month, an Afghan soldier shot and killed four unarmed French troops at a base in eastern Afghanistan.

Friday's shooting in Sari Pul province in northern Afghanistan resulted from an unfortunate misunderstanding, said Sayed Jahangir, the deputy police chief for the province.

Afghans guard outside the perimeter of the base and Americans guard inside. Jahangir said that the Afghan guard, Abdul Rahim, wanted to go into the base and started arguing with the American at the door. Rahim did not raise his weapon, but the American thought he was about to do so and fired, Jahangir said.

"Our initial reports show that the American thought he was acting in self defence," Jahangir said. Rahim was a private guard, not an Afghan soldier or policeman, Jahangir said.

U.S. forces were "aware of an incident in northern Afghanistan" and were investigating, said U.S. military spokesman Lt.-Col. Jimmie Cummings. He declined to provide further details.