But General David Petraeus insisted the immediate focus had to be on maintaining the improved security situation in Basra.

Speaking after talks with Gordon Brown in Downing Street, Gen Petraeus said they had considered the issue of how long the UK force strength needed to remain at around 4,000.

"The answer is right now we don't know," he said. "How long that stays is something we need to work out in the course of the next month or two as we look at the so-called troop-to-task analysis."

The Prime Minister had set out plans to reduce British troop levels in Iraq to 2,500 by this month. However, last month the Government admitted the reduction had been postponed indefinitely due to increased violence.

In March, the military were criticised in some quarters for failing to provide enough assistance to Iraqi forces during a major offensive against Shia militias in Basra.

On Thursday Gen Petraeus praised the "key role" being played by Britain in the troubled country, but added: "The key right now, frankly, is to sustain what has been achieved in Basra, to help the Iraqis do that and then over time - as I've said, the next two months or so - work out where the path from there will be."

The meeting at No 10 - which was also attended by US ambassador to Iraq Ryan Crocker - lasted for around an hour.