President George W. Bush said on Thursday the Pentagon would halt troop reductions in Iraq this summer but that deployments would be shortened to ease the stress on the military.

Mr Bush was endorsing the recommendation of General David Petraeus, the US commander in Iraq, to have a 45-day pause following the departure of the combat forces that made up the “surge”. He vowed to give the four-star general “all the time he needs” to assess the impact unwinding the surge would have on security in Iraq.

Earlier this week Gen Petraeus frustrated lawmakers by refusing to say whether he expected further troop reductions this year. On Thursday, however, Robert Gates, US defence secretary, said he hoped the Pentagon could withdraw more troops later this year.

“I do not anticipate this period of review to be an extended one, and I would emphasise that the hope, depending on conditions on the ground, is to reduce our presence further this fall,” he said. “But we must be realistic. The security situation in Iraq remains fragile.”

But Mr Gates also conceded that he no longer believed it was possible to reduce troop levels to 100,000 this year, as he had hoped last September.

Mr Bush also bowed to the concerns of the joint chiefs of staff – the heads of the army, marine corps, navy and air force – by announcing that soldiers deploying to Iraq and Afghanistan after August would face 12-month tours, instead of the current 15 months.

Admiral Mike Mullen, the chairman of the joint chiefs, who joined Mr Gates before Congress, expressed support for the pause. He also rejected criticisms from lawmakers that the pause was “open-ended”.

“It’s not a blank cheque. It’s not an open-ended commitment of troops,” said Adm Mullen. “It’s merely recognition of the fact that war is unpredictable.”

While violence has declined dramatically since the surge took effect last June, it has increased again recently following a military offensive against Shia militia by Nouri al-Maliki, the Iraqi prime minister.

Gen Petraeus on Thursday expressed concern that Mr Maliki’s operation could cause the ceasefire declared last year by Moqtada al-Sadr, the radical Shia cleric who heads the Madhi army, to “fray”.

US commanders view that ceasefire, which has been tested in recent weeks, as one of the pillars supporting the improvement in security over the past year.

“There has to be a very, very sensitive approach as this goes forward, to make sure that folks don’t feel like they’re backed into a corner from which there’s no alternative,” said Gen Petraeus.

In pressing his case for the war, Mr Bush said success in Iraq would “bring us closer to our most important goal – making the American people safer here at home”. But earlier this week Gen Petraeus responded to a question about whether the campaign in Iraq was making the US safer by saying that “ultimately, it can only be assessed by history”.

US military commanders have repeatedly voiced concern that Iran was helping foment violence in Iraq by supporting and training so-called “special groups” of Shia militias.

Mr Bush on Thursday said the Iranian regime had a choice between developing peaceful relations with Iraq or continuing to “arm and train and fund illegal militant groups which are terrorising the Iraqi people and turning them against Iran”.