Four Afghan police officers have been killed and two civilians injured after a roadside bomb hit a police vehicle in the southern city of Kandahar, police have said.

The bomb, hidden under a bridge, was remotely detonated on Sunday when a police ranger was passing, Fazel Ahmad Sherzad, the deputy provincial police chief of Kandahar, said.

"Four of our policemen were killed on the spot," Sherzad said, blaming the attack on Taliban fighters.

Aziz Ahmad, a resident of Kandahar city, once the stronghold of Taliban, said that two civilians were also wounded in the blast and were rushed to hospital by relatives.

The Taliban has so far not claimed responsibility, but the attack bore their hallmarks as the fighters rely heavily on roadside bombings as part of their campaign against the Afghan government and international troops.

The attack in Kandahar, the biggest city in the southern region, comes three days after a suicide bombing killed four people and wounded more than 20 others.

Meanwhile, Hamid Karzai, the Afghan president, has ordered an investigation into an incident in which Afghan police forces killed seven civilians, mistaking them for anti-government fighters in Kandahar's Spin Boldak district, according to a statement from the presidential palace.

The seven civilians, who were collecting firewood on a mountain near the Pakistani border, were shot dead on Friday morning.

Six police, including their unit commander, have been detained.