BASRA, Iraq, March 30 (Reuters) - British forces set up a checkpoint at a bridge outside Basra on Sunday, witnesses said, but a spokesman said they had no plans yet to retake Iraq's second city, scene of a government crackdown on Shi'ite fighters.

Witnesses said British troops in 8-10 armoured vehicles had set up the checkpoint at the Zubair bridge south of the city and were checking cars heading into Basra.

Britain withdrew its forces from Basra in December and a remaining contingent of 4,100 troops has largely kept to a base outside the city during the past six days, in which government troops have battled militia loyal to cleric Moqtada al-Sadr.

"We've had ground forces outside the wire assisting Iraqi forces. There are no British ground forces inside the city of Basra," spokesman Major Tom Holloway said by telephone. "As yet there is no intent to push British armour into the city."

The U.S. military has confirmed that it has special forces operating alongside Iraqi troops in what began as an Iraqi-led operation. They have called in air strikes from U.S. warplanes.

Holloway said British forces have also fired artillery in support of British ground forces. (Reporting by Aref Mohammed in Basra and Peter Graff in Baghdad; writing by Peter Graff; editing by Sami Aboudi)

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