The commander of US forces in Iraq today told the Senate that troop reductions should be indefinitely put on hold for fear of undoing recent security advances.

In testimony to the Senate Armed Services Committee General David Petraeus said that a freeze should be imposed from July after the 20,000 additional troops deployed during last year’s surge had been withdrawn.

A 45-day pause would provide an opportunity to evaluate the impact of further withdrawals on the security situation, he said. That period would need to be followed by an open-ended "assessment" phase.

As he spoke fighting raged in Baghdad’s main Shia militia stronghold, while the deaths of 10 American soldiers in a variety of attacks over a three-day period served as a sobering reminder of the cost of the war to US lawmakers.

During the hearing the general was twice interrupted by protestors, one of whom was removed from the chamber after repeatedly chanting, "Bring them home," prompting applause from some sections.

Gen Petraeus told the hearing that significant progress had been made since the beginning of the troop build-up 15 months ago but cautioned that it remained “fragile and reversible”.

He warned that a rushed withdrawal would allow al-Qaeda and Iran to regain their strength and influence in the country. Iranian support for Shia militia was “the greatest long-term threat to the viability of a democratic Iraq”, he added.

The general also pointed to the recent flare-up of violence by Shia militants in Basra and Baghdad and the operation by Iraqi security forces, which he said had been "disappointing" in logistical terms and demonstrated the need for continued US support.

President Bush has already said he will accept the general's recommendations and is expected to formally endorse it on Thursday.

Senator Carl Levin, the Democratic chairman of the committee, responded by claiming the recommendation amounted to an “open-ended pause” that would remove pressure on the Iraqi government to take responsibility for their country.

“What you’ve given to your chain of command is a plan which has no end to it,” he said.

Under examination from the Senator, Gen Petraeus admitted that he had no estimate of when further withdrawals might be able to take place.

US Ambassador to Iraq Ryan Crocker, who also testified, gave details of an agreement to extend the US presence in Iraq beyond the end of this year, when the current United Nations mandate expires.

It would not, however, allow the establishment of permanent US bases in the country, he stressed, saying that he anticipated the agreement would "explicitly foreswear" such a move.

Earlier, Senator John McCain, the presumptive Republican nominee whose support of the US presence in Iraq has been a key plank of his campaign, had delivered an upbeat assessment of the security gains delivered by the surge.

He insisted the country had “come a long way” since General Petraeus’ last testimony in September and even further since the beginning of the surge in January 2007 when the country was on the brink of a “full-blown civil war.”

In a sideswipe at President Bush, however, he blasted the “four years of mismanaged war” that had brought the country “almost to the point of no return”.

“While the job of bringing security to Iraq is not finished… we are now longer staring into the abyss of defeat and we can now look forward to the prospect of success,” he said.

Senator Hillary Clinton, the Democratic presidential candidate, offered a far more scathing appraisal.

Describing the security situation as “tenuous”, she claimed the surge had not resulted in the political progress it had been intended to facilitate. The general himself had admitted progress had been “insufficient” in a newspaper interview just a month ago, she noted.

Arguing for a “responsible and carefully planned withdrawal”, she said it might well be irresponsible to continue a policy that kept failing to deliver the results the American people had been promised.

The ongoing presence in Iraq was also detracting from Afghanistan and other international problems, Mrs Clinton said.

The Democratic presidential frontrunner, Senator Barack Obama, was due to question the general at a second committee hearing later today.

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