British special forces are still in Mazar-e-Sharif
They guided American warplanes to target a fort in the city of Mazar-e-Sharif, where inmates were under siege.
Earlier four British special forces servicemen were airlifted back to the UK after they were injured in Afghanistan.
They are being treated at a hospital in Birmingham.
The soldiers - the first UK casualties of the conflict - were wounded while operating with US special forces.
Their injuries are not said to be life-threatening, although one was wounded more seriously than the others.
No Survivors
The Taleban prisoners at the Qala-e-Jhangi fortress had resisted Northern Alliance troops, despite being subjected to heavy bombardment by US aircraft for three days. The fighting ended on Tuesday.
The alliance had detained about 500 non-Afghan Taleban prisoners in the fortress after they surrendered at Kunduz.
A spokesman for the alliance said all those that were in the fortress on Tuesday had been killed.
Confirmation that British troops had taken part in the operation came after UK Defence Secretary Geoff Hoon announced that only 400 British troops would be kept on a heightened state of readiness to go to Afghanistan.
The majority of the 6,000 troops and supporting personnel on 48-hours stand-by for Afghanistan had had their notice to move relaxed to one week, he said.
Just troops from the 2nd Battalion Parachute Regiment and the 16 Air Assault Brigade will be kept on heightened readiness for action, following a reassessment of the likely operational needs.
The troops had been intended to form part of a multi-national "stabilisation" force promoted by Britain but have been put on hold in the face of a lack of enthusiasm by the US and outright opposition from the Northern Alliance.
The hunt for Osama Bin Laden was also gathering pace with the deployment of hundreds of US marines in the Taleban's southern heartland, the first serious involvement of western troops.
Mr Hoon said a small force of 100 Royal Marine commandos and Special Boat Service troops securing the Bagram airbase near Kabul had been reinforced over the weekend.