Afghan and foreign troops were hunting on Saturday hundreds of prisoners, including militants, who escaped the main prison in southern Kandahar city after a raid by Taliban insurgents, the government said.

Separately, an explosion killed four soldiers from the U.S.-led force during an operation in the western province of Farah on Saturday, the single bloodiest toll among foreign troops in one day in recent weeks in Afghanistan.

Authorities have also launched a probe to find out if any security officials were involved in the commando-style attack on the Kandahar prison by several dozen Taliban fighters under darkness on Friday.

So far none of the prisoners have been tracked down, deputy justice minister Mohammad Qasim Hashimzai, told Reuters.

"It was a very unprecedented attack and together with foreign forces, an operation has been launched to track down and arrest the prisoners."

Hashimzai said some 1,000 inmates, including up to 400 Taliban, were held in the prison before the attack.

He could not say how many had managed to escape, adding there were casualties among police, the Taliban and prisoners from a clash following the attack which began with a suicide bomber driving a truck into the jail gate.

Several dozen Taliban, armed with rocket propelled grenades and assault rifles then stormed the mud-built compound and started to free the prisoners which apart from militants included women and suspected criminals.

"We are trying to find out that if there was any inside help," Hashimzai said.

A local politician said 15 policemen were killed in the Taliban storming of the prison and subsequent clashes. He did not have more details.

NATO-led troops were supporting Afghan security forces in cordoning off the area in the hunt for the prison inmates, an alliance spokesman in Kabul said.

SEARCHES

In Kandahar city, Afghan forces were checking vehicles and motorcyclists on roads. Some houses, where authorities suspected some escapees had hidden, were also searched, residents said.

Dozens of police and army soldiers were deployed outside the badly damaged prison.

They ordered vehicles to move away from the road which is only metres away from the jail. A pile of rubble caused from the collapse of two towers of the jail along with its broken gate could be seen.

The impact of the blast had badly damaged kiosks and shops across the road. "I was packing to finish for the day and all of a sudden heard a very, very loud explosion and then a huge flame," said Abdul Qodous, a shopkeeper.

"Then I started to run away,".

Some high ranking Taliban field commanders were also among those who have managed to escape, a politician said from Kandahar, the birth place and the main stronghold of the Taliban who were ousted from power in 2001.

A Taliban spokesman, Qari Mohammad Yousuf, speaking to Reuters from an unknown location by phone, said all Taliban former prisoners had arrived at their "safe destinations".

The U.S. military has handed over an unspecified number of suspected Taliban fighters to Afghan custody under a programme agreed last year to transfer all Afghan prisoners from U.S. detention.

The attack came a day after international donors pledged more than $20 billion (10 billion pounds) for Afghanistan where over 60,000 foreign troops led by NATO and the U.S. military as well as some 150,000 Afghan forces are trying to defeat the Taliban-led insurgents.

Frustration is high among Afghans over growing insecurity, lack of economic development, corruption and with hundreds of civilian deaths by foreign troops while hunting the Taliban.

On Saturday hundreds of people staged a protest in the southeastern province of Paktia, accusing U.S.-led coalition troops of killing civilians in an air strike this week.

Afghan officials say two women were killed in late Thursday's raid and that the rest of the victims were militants.