General David Petraeus called the achievements of the US “surge” in Iraq “fragile and reversible” and said that Iran now posed the greatest long-term threat to achieving stability in the country and the region.

In a sobering report the US commander in Iraq told two Senate committees – including the three presidential candidates vying to become his next commander-in-chief – that the addition of 30,000 US troops last year “has achieved progress, but that progress is reversible”. He called for what appears to be an open-ended halt this July to the withdrawal of US forces.

In testimony that singled out Tehran as a malevolent force that was arming militant factions in Iraq, he said: “The situation in certain areas is still unsatisfactory and innumerable challenges remain.”

Later he added: “We haven’t turned any corners, we haven’t seen any lights at the end of the tunnel.”

The general’s strategy to maintain high troop levels was criticised heavily by Democrats. He also conceded that a botched assault by Iraqi troops against the militia of Shia cleric Hojatoleslam Moqtada al-Sadr in Basra last month was badly planned and executed.

General Petraeus appeared on Capitol Hill after the deaths of 11 US military personnel in Iraq in the past 48 hours – and the recent milestone of 4,000 American dead – and at a time when a significant majority of Americans believe that the war was a mistake. Several protesters were ejected during his testimony. In the hours before he spoke, fighting raged in Baghdad’s main Shia militia stronghold of Sadr City, the bulwark of Hojatoleslam al-Sadr’s al-Mahdi Army. Another 12 Iraqis were killed in a third day of clashes with government forces and US troops.

General Petraeus said that the recent violence in Basra highlighted the role Iran played in funding and training Shia militia through cells the US military calls “special groups”. He said: “Unchecked, the special groups pose the greatest threat to the viability of a democratic Iraq.”

Ryan Crocker, the US Ambassador to Iraq, who also gave testimony, said that Tehran was attempting a “Lebanisation” of Iraq. Contrary to reports, Mr Crocker said, an agreement now being negotiated with the Government of Nouri al-Maliki “will not establish permanent bases in Iraq – we anticipate that it will expressly forswear them”.

He added that the agreement “will not specify troop levels, and it will not tie the hands of the next president”.

General Petraeus, whose recommendations will almost certainly be followed by President Bush, called for the reduction of US troops to the presurge level by July, followed by a 45-day “period of consolidation and evaluation”.

He then recommended a subsequent, unspecified period to “determine when we can make recommendations for further reductions”. He refused to give any timetable for withdrawals, and his advice means that the next president will almost certainly inherit a situation in Iraq with at least 130,000 US troops deployed.

Carl Levin, the Democratic chairman of the Senate Armed Services Committee, accused General Petraeus of advocating an “indefinite pause” that would be the “next page in a war with no exit strategy”. Hillary Clinton suggested that it was “irresponsible to continue a policy that has not produced results time and time again and at such tremendous cost”. John McCain, the Republican nominee-elect, said that to withdraw troops would “abandon Iraq to civil war”.

In a later hearing, Barack Obama said that the US should talk to Iran as part of a “diplomatic surge”. He repeated his call for a withdrawal timetable, adding that “nobody is asking for a precipitous withdrawal”.

Asked by John Warner, a Republican, if the Iraq war had made America safer, General Petraeus failed to answer directly but eventually said: “I do believe it is worth it.” To the same question, Mr Crocker said: “Al-Qaeda is our mortal and strategic enemy. To the extent that its capabilities have been diminished in Iraq, it makes our country safer.”

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