About 15 by-standers were also injured in the attack on NATO-led forces

travelling on a main road near the police headquarters in Lashkar Gah, capital of the volatile province of Helmand.

"Four civilians were killed and 15 wounded," provincial spokesman Omar

Zuwak told AFP. He was unable to give details about any foreign casualties.

NATO's International Security Assistance Force (ISAF) did not immediately provide any information on the attack.

About 87,000 NATO-led troops are deployed in Afghanistan to fight an

Islamist insurgency that erupted after the hardline Taliban regime was ousted from power in 2001.

Foreign troops are withdrawing from the battlefield as Afghan security

forces step up to take responsibility for thwarting the Taliban, who are

strongest in the southern provinces of Helmand, Kandahar and Uruzgan.

Afghan and international leaders are also trying to seek a peace deal with the Taliban to halt the 12-year conflict before a presidential election in April and the departure of all NATO combat troops by the end of the year.

No group immediately claimed responsibility for Wednesday's attack but

suicide bombings are a hallmark of the Taliban. Civilian by-standers are often caught by the explosions.

According to a recent United Nations report, civilian casualties in the

Afghan war rose 23 percent in the first half of this year due to Taliban

attacks and increased fighting between insurgents and government forces.

Elsewhere in Afghanistan, four drivers were killed and 40 trucks set ablaze when a rocket was fired at a parking lot in Farah province. The provincial spokesman blamed the Taliban.