US President George W Bush, assailed by sagging poll numbers and criticism from anti-war protesters camped outside his ranch, has called for Americans to show resolve and brace for additional sacrifice in Iraq.

Mr Bush, who personally intervened this week with a key Shiite leader in a bid to broker a deal on Iraq's constitution, said Iraqis were "making the tough choices and compromises necessary for a free and peaceful future."

With almost 1,900 US troops dead in the Iraq conflict, Mr Bush is under mounting pressure from critics to finish training a new Iraqi security force and bring the soldiers home.

Bush supporters and anti-war protesters, including relatives of soldiers who died in Iraq, were slated to face off this weekend with rallies near the president's 648-hectare Crawford ranch, where he has been spending much of August on vacation.

Cindy Sheehan, whose son died in Iraq in April 2004, has been holding a vigil outside the ranch seeking another meeting with him to press for the quick withdrawal of US troops.

She will be countered by other soldiers' families who back the US action in Iraq.

In his weekly radio address, Mr Bush acknowledged the job for US soldiers was not yet done.

"Our efforts in Iraq and the broader Middle East will require more time, more sacrifice and continued resolve," he said.

Mr Bush has said withdrawing now would only embolden insurgents who have sought to derail the drafting of an interim constitution.

"When Iraqi forces can defend their freedom by taking more and more of the fight to the enemy, our troops will come home with the honour they have earned," he said.

Mr Bush praised the efforts of Iraqi leaders to agree on principles to guide their country's future, despite religious and other divisions.

"What is important is that Iraqis are now addressing these issues through debate and discussion - not at the barrel of a gun," he said.

Mr Bush's message was part of renewed push to counter critics of his Iraq war policy and boost his standing in the polls.

The latest Gallup survey showed that just two-in-five Americans approved of the job he was doing while 56 per cent disapproved of his performance.