Condoleezza Rice said Washington saw the task of stabilising Iraq as not simply improving security within its borders but "to begin to have American forces in lower numbers turn to other responsibilities".

She said "the territorial security of Iraq" with regard to its neighbours - especially Iran - was among them.

"Iran is a very troublesome neighbour," she told NBC's Today show. Tehran has said it is prepared to "fill the vacuum" if the US leaves Iraq. "That is what is at stake here," Ms Rice added.

The secretary of state's comments came a day after General David Petraeus, the top US commander in Iraq, and the US ambassador to Baghdad, Ryan Crocker, finished two days of gruelling testimony on Capitol Hill.

Gen Petraeus said he would recommend that the extra 30,000 troops sent to Iraq this year be withdrawn next summer, bringing numbers down to 130,000 - the same as at the start of the year.

In its first response to the Petraeus-Crocker testimony, Iran said the troop reduction would not "save America from Iraq's swamp" and repeated its call for a full US withdrawal.

The US president, George Bush, is set to accept the recommendation in a 15-minute speech tomorrow.

However, the prospect of a large US military presence for the foreseeable future has angered Democrats and unsettled more Republicans.

Nancy Pelosi, the Democratic House speaker and a strong opponent of the war, met Mr Bush yesterday and has sharply criticised the president's approach as "an insult to the intelligence of the American people".

Ms Pelosi said everything she had heard "sounds to me like a 10-year, at least, commitment to an open-ended presence and war".

Mr Bush's determination to keep 130,000 troops in Iraq at least until next summer could lead to more wobbling in the Republican ranks.

Senator Elizabeth Dole, who has stuck by the administration on Iraq, yesterday said she would now support "what some have called action-forcing measures".

Congressional opponents of the conflict still lack enough votes to cut off funding for it or set deadlines for a US withdrawal.

However, fresh moves are afoot to pass legislation calling for a change of the US mission away from combat to training that would reduce the troop presence to between 50,000 and 60,000 soldiers.

Meanwhile, the Democratic senator, Barack Obama, called for the withdrawal of all US combat troops from Iraq by the end of next year.

Mr Obama, one of the Democratic frontrunners for the White House, will urge the US to adopt a clear timetable starting with an immediate reduction in combat brigades until they are all out by the end of 2008.

"Our drawdown should proceed at a steady pace of one or two brigades each month," he is expected to say in a speech at Ashford University in Clinton, Iowa. "If we start now, all of our combat brigades should be out of Iraq by the end of next year."

By arguing that only combat brigades should be withdrawn - there are 20 in Iraq, including five sent in January - Mr Obama appeared to suggest that other US troops could remain.

In Baghdad, the Iraqi national security adviser, Mowaffaq al-Rubaie, said foreign troop levels in Iraq could drop to less than 100,000 by the end of 2008 if the country's own forces were ready and security threats had diminished.