The White House suffered the setback when a roadside bomb killed Abdul-Sattar Abu Risha and two of his bodyguards near the tribal leader's home in Ramadi, Anbar's provincial capital.

Abu Risha was killed 10 days after meeting Mr Bush during his surprise visit to Iraq. He led the Anbar Salvation Council, an alliance of clans that turned against al-Qaida and went over to the Iraqi government and the US military.

US officials have described the rift between Sunni tribes and al-Qaida in Anbar province, in western Iraq, as one of the most significant developments of the war.

From 2003 until last year, Anbar was the heartland of the Sunni Arab insurgency and the most dangerous part of Iraq.

Al-Qaida once controlled large swathes of the province, but angered local tribal leaders with its indiscriminate killing of civilians and harsh interpretation of Islam.

Pentagon officials said the assassination would be a huge blow to US efforts in Iraq because it would send a message to others cooperating with US-led forces or thinking about cooperating against al-Qaida.

During a visit to the al-Asad air base this month, Mr Bush hailed Abu Risha and others "who have made a decision to reject violence and murder in return for moderation and peace".

The latest setback to the US military effort came just hours before Mr Bush was due to make hs latest appeal for patience to the US public.

In a 15-minute speech, he is expected to ask the American public for more time for his strategy to work and cite the threat from Iran as a reason for a heavy US troop presence in Iraq.

While pointing to what he sees as progress on security, he will insist that any troop reductions will depend on continued improvement. Underscoring the importance of Mr Bush's televised address, the White House said it had gone through more than 20 drafts.

Without the votes to push for a timetable for withdrawal, congressional Democrats are expected to focus on efforts to change the emphasis of the US military in Iraq from combat to training.

However, they appear resigned to the fact that Mr Bush will keep a large number of troops in Iraq for the rest of his presidency.

Mr Bush's speech comes after two days of congressional testimony from Genreal David Petraeus, the top US commander in Iraq, and the ambassador, Ryan Crocker.

Gen Petraeus recommended a withdrawal of 30,000 troops by next summer, bringing troop numbers back to 130,000 - the same level as before the "surge" began in February.

Mr Bush is expected to accept that recommendation while maintaining a hard line on Iran. Gen Petraeus said a further decision on troop levels would have to wait until next March.

A US general today accused Shia militants backed by Iran of executing a deadly rocket attack on a US military headquarters in Iraq.

One person was killed and a dozen injured in the incident, which happened when a 240mm rocket hit Camp Victory, a sprawling base near Baghdad international airport, two days ago.

The US claims such rockets are supplied to Shia extremists by Tehran.

Major General Kevin Bergner said the rocket was launched from the Rasheed district of west Baghdad, which he said had been infiltrated by the Mahdi army militia of the radical cleric Moqtada al-Sadr.

"The attack used a 240mm rocket, which is a weapon that these groups have received from Iranian sources in the past and recently used in other attacks against coalition forces," Gen Bergner told a news conference in Baghdad.

As part of its campaign against Iran, the US is starting a drive for a third round of sanctions against the country.

Iran has emerged as an increasingly important rationale for the US to maintain a significant presence in Iraq. Mr Crocker told the Washington Post that Iran now had a "fairly aggressive strategy" on the ground in the country.

He said Tehran's increased support of extremist militias contributed to the near unravelling of Iraq last year, adding that it was now trying to create a force like Hizbullah, the Iranian-backed Shia force in Lebanon, to advance its long-term interests in Iraq.

In a briefing at the National Press Club in Washington, Gen Petraeus said arms supplies from Iran, including 240mm rockets and explosive devices, "contributed to a sophistication of attacks that would by no means be possible without Iranian support ... the evidence is very, very clear".