The senior US commander in Iraq has called for a pause in troop withdrawals from the country after July in order to assess last year's so-called troop "surge".

Testifying in front of US Congress, General David Petraeus warned on Tuesday that "significant" military gains from the "surge" were "fragile and reversible".

He told the Senate Armed Services Committee that the current withdrawal of five combat brigades should continue before a 45-day period of "consolidation and evaluation" is taken.

Ryan Crocker, the US ambassador to Iraq, also testified at the hearing, part of a two-day Congress session on Iraq.

Proceedings were interrupted on several occasions by heckling from anti-war protesters, one of whom shouted "bring them home", referring to US troops, before being escorted from the room.

Criticism of Iran

General Petraeus said that after the 45-day period the US would "commence a process of assessment to examine the conditions on the ground and, over time, determine when we can make recommendations for further reductions".

The current troop drawdown would reduce troop levels from about 158,000 to 140,000.

He also continued to criticize Iraq's neighbor, Iran for supporting and training Shia fighters in Iraq through cells the US army described as "special groups".

"Unchecked, the special groups pose the greatest long-term threat to the viability of a democratic Iraq,'' Petraeus said.

Recent violence in Iraq between Shia militias and Iraqi security forces has left hundreds of people dead, while on Tuesday Muqtada al-Sadr, the Shia leader of the Mahdi army, threatened to lift a ceasefire he put in place last August.

General Petraeus also criticized the recent Iraqi operation against Shia militias in the southern city of Basra as "not adequately planned or prepared".

Meanwhile, Crocker said that the US is negotiating a long-term agreement with Iraq that would give a legal framework for US troops to remain in the country.

The move has sparked criticism from some Democrats who say the move would shift the burden of the war on to the next US administration.

"The agreement will not specify troop levels, and it will not tie the hands of the next administration," he said.

"[The next US president] arrives in office with a stable foundation upon which to base policy decisions.''

Democratic anger

The spike in violence in Iraq has also thrust Iraq to the forefront of campaigns for the US presidential election in November.

Both the White House and members of the Republican party say the "surge" successfully lessened violence and argue a pause in troop withdrawals to pre-surge levels provides the means to assess the results of the "surge".

John McCain, the presumptive Republican candidate, told the Senate Armed Services committee ahead of Petraeus's testimony on Tuesday that "much needed to be done but said it was possible to "talk with real hope and optimism about the future of Iraq and the outcome of our efforts there.''

However, several Democrats criticized Petraeus' plan to halt withdrawals, saying the surge has not yielded sufficient political progress in Iraq and no end to the conflict is foreseeable.

Senator Carl Levin, chairman of the Senate Armed Services Committee, said that General Petraeus had given his chain of command "a plan which has no end to it".

And both Democratic presidential hopefuls, who were present at General Petraeus's testimony, said the current Iraq strategy was not working, with Hillary Clinton telling General Petraeus it was time to withdraw US troops, saying the situation in Iraq remained tenuous.

"It might well be irresponsible to continue the policy that has not produced the results that have been promised time and time again," she said.

Meanwhile Democratic presidential hopeful Barack Obama told NBC news channel on Tuesday that "the height of irresponsibility was going in [to Iraq]."

"It compounds the irresponsibility if all we're doing is simply moving the goal posts."

PHOTO CAPTION

US Commander in Iraq General David Petraeus gives a testimony about the Iraq war before the Senate Armed Services Committee on Capitol Hill in Washington April 8, 2008.

Al-Jazeera