(Updates with more detail, colour)

BASRA, Iraq, April 5 (Reuters) - Two roadside bombs hit British forces in a Shi'ite militia stronghold in the southern Iraqi city of Basra on Thursday.

One of the bombs left a large crater in the road that was at least a metre deep and several metres wide. Iraqi police said British troops had stormed a checkpoint close to the scene of the blasts shortly afterwards and disarmed the police there.

British military spokesmen in Basra, the hub of Iraq's main oil fields, had no word on casualties. Two British soldiers have been killed in separate small arms attacks this week.

One resident said he had seen at least one armoured vehicle ablaze, possible a Warrior infantry fighting vehicle, which British forces typically use in Basra.

"We heard two explosions that shook the house. I went out and saw one armoured vehicle that was completely destroyed and another with less damage. I saw some soldiers being taken away, but I don't know how many," he said, declining to give his name.

British military spokeswoman Katie Brown said a roadside bomb hit British troops at 2 a.m. (2200 GMT Wednesday). She gave no further details. A second spokesman, Lieutenant-Colonel Kevin Stratford-Wright, confirmed only that there had been a "serious incident" in the night.

The incident took place in Hayaniya, a slum area on the northwestern outskirts of Basra that is a stronghold of the Mehdi Army militia of radical Shi'ite cleric Moqtada al-Sadr.

In mid-February, British troops clashed with insurgents armed with machineguns and rocket-propelled grenades in the area, killing at least three gunmen.

British Prime Minister Tony Blair said in February Britain would begin withdrawing a quarter of its 7,000 troops -- who are stationed mainly in and around Basra city -- in the coming months, paving the way for Iraqis to take full control of Basra province.

The commander of British forces said last month the greatest obstacle to Iraqis taking control was the perception that the region was lawless, fuelled by the high number of attacks on British troops.

At least 136 British soldiers have been killed in Iraq since the U.S.-led invasion in 2003.

(Additional reporting by Aseel Kami, Yara Bayoumy, Ross Colvin in Baghdad)

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