By Aref Mohammed

BASRA, Iraq, April 5 (Reuters) - Four British soldiers and a civilian interpreter were killed in a roadside bomb blast that destroyed their Warrior armoured fighting vehicle in the Iraqi city of Basra on Thursday, the British military said.

The deaths brought to six the number of British soldiers killed in Iraq this week, making it one of the deadliest for British forces since the U.S.-led invasion in 2003.

Separately, four American soldiers were killed in two separate roadside bomb attacks in and around Baghdad on Wednesday, the U.S. military said.

The attacks follow a relatively quiet period in Baghdad, where U.S. and Iraqi forces have deployed thousands more troops to enforce a new security crackdown.

The U.S. military said it was also looking into reports that a military helicopter had been brought down south of Baghdad. Witnesses said heavy gunfire forced down what they said was an Apache attack helicopter.

British military spokesman Lieutenant-Colonel Kevin Stratford-Wright said a British force had been attacked overnight with roadside bombs, rocket-propelled grenades and small arms on the western outskirts of Basra, the hub of Iraq's main oil fields, as it returned from an operation.

A fifth soldier was seriously wounded.

The nationality of the interpreter was unclear. He was initially thought to be Iraqi.

The attack took place near 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.

The main blast left a crater in the road at least a metre deep and several metres across.

"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," said one resident who declined to give his name.

British Prime Minister Tony Blair said in February that 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 eventually take full control of Basra province.

At least 140 British soldiers have now been killed during the Iraq war.

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

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