Speaking at the end of a summit of world leaders, Berlusconi said Italy was a prime target for Islamist extremists thanks partly to its troop deployment in Iraq.

But he shrugged off calls from back home, including from within his own government, to speed up the troop pullout following the deadly bomb attacks in London on Thursday. "We have to fulfil our commitments and cannot leave the job half done," Berlusconi told a news conference.

"As far as our troop withdrawal goes, the situation has not changed. We will begin, as I have already announced, a partial withdrawal of around 300 troops in September," he added.

Italy has some 3,000 troops in Iraq, the fourth largest foreign contingent there after the United States, Britain and South Korea.

Earlier on Friday, a group claiming links to al Qaeda posted a message on the Internet threatening to attack Rome to punish Italy for supporting the United States.

Italy dispatched more plain-clothes police to guard public transport networks following the London bombings and increased security at airports. Berlusconi said police would also review security at U.S. and British interests around Italy.

"There is a war on against the West by people who want to show that our civilization is evil," said Berlusconi, adding that British Prime Minister Tony Blair, President Bush and he were directly in the firing line.

"Even intelligence (reports) from other countries show the three Bs, Bush, Berlusconi and Blair, are considered the most exposed to this type of risk," he added