The US administration has indicated that it may have to attack Iraq without the assistance of British troops.

Speaking at the Pentagon, US defence secretary Donald Rumsfeld told reporters: "Their situation is distinctive to their country and they have a government that deals with a parliament in their distinctive way.

"And what will ultimately be decided is unclear as to their role; that is to say, their role in the event a decision is made to use force."

The statement triggered a frantic bout of phone calls between Washington and London.

In a written statement, Rumsfeld clarified his position, saying: "In my press briefing today, I was simply pointing out that obtaining a second United Nations security council resolution is important to the United Kingdom and that we are working to achieve it."

Observers suggest the move comes amid growing US unease over Tony Blair's failure to secure support for his war plans.