George W Bush last night faced down his critics on Iraq, warning that the Middle East would be "dramatically transformed" in a way that would endanger the civilised world if extremist forces drove America from the region.

In a speech to army veterans in Nevada, the US President declared: "America will not abandon Iraq in its hour of need."

Describing the battle as one between the forces of extremism and freedom, Mr Bush said: "Either our enemies advance their interests in Iraq or we advance our interests. The most important and immediate way to counter the ambitions of al Qaeda, Iran and other forces of instability and terror is to win the fight in Iraq."

Mr Bush is coming under increasing pressure ahead of a key report on the state of play in Iraq from General David Petraeus, the US commander on the ground, which is likely to determine the fate of America's mission in Iraq.

As Mr Bush defended his policy, David Miliband, the Foreign Secretary, appeared to pave the way for Gordon Brown to announce the beginnings of a British withdrawal later this year by insisting any such decision would be made independently of Washington.

While the Prime Minister has rejected demands for a fixed timetable for withdrawal, the tone and nuanced language increasingly appear to be pointing in one direction.

To add to Washington's frustration, in Tehran President Mahmoud Ahmadinejad boldly declared that US political influence in Iraq was "collapsing rapidly".

In another worrying development, retired Vice Admiral John Scott Redd, the head of the National Counter-terrorism Centre in America, claimed that US authorities had "very strong indicators" al Qaeda was planning to attack the West.

Elsewhere in Afghanistan, a suicide bomber killed three Nato soldiers and wounded six others in the eastern province of Patkia as they worked on a bridge construction.