They are under investigation. They pretend they were chasing a group of thieves, but the reality is that they wanted to sabotage the security situation in Kabul

Afghanistan's Interior Ministry official

They were attacked simultaneously in the dark by a group of about 30 men, armed with AK-47 automatic rifles.

The British soldiers, from the Royal Anglian Regiment, returned fire and captured seven men, five of whom are reported to have been wearing local police uniforms. No casualties were reported on either side.

A senior Afghan official said the seven captured men were members of the Afghan interim administration 's security force, after the British troops handed them over to the local Interior Ministry.

"They are under investigation. They pretend they were chasing a group of thieves, but the reality is that they wanted to sabotage the security situation in Kabul. That was for sure their intention," General Deen Mohammad Joorat told the Reuters news agency.

Lawlessness

British troops form the core of the Isaf, trying to build stability in Kabul.

Afghanistan urgently wants to rebuild its own army and police force

The BBC's Andrew Harding, reporting from Kabul, says the two patrols were about 300 yards apart in a western area of the city when they came under attack.

Isaf spokesman Lieutenant-Colonel Neal Peckham said it was not clear whether the attack was deliberate or whether the two groups had clashed accidentally.

"At the moment we are still trying to get to the bottom of exactly what this force represented".

He added that either way, the Afghans appeared to have been involved in some form of lawless activity.

Our correspondent says that Afghanistan is still crowded with different armed groups, some controlled by warlords, others now supposedly confined to barracks.

There has been speculation that some of these men are turning to crime to supplement their meagre wages.

There are also fears that forces hostile to the new interim government may be trying to destabilise Kabul, either by attacking Isaf troops or other targets.

Reports from the central province of Wardak say nine fighters have been killed in clashes between two local warlords, one of them loyal to Afghanistan's interim administration.

The Afghan Islamic Press news agency in Islamabad says 12 people were injured in the fighting.