President George W. Bush on Thursday ordered gradual troop reductions in Iraq but defied calls for a dramatic change of course, telling war-weary Americans the U.S. military role there will stretch beyond his presidency.

Trying to secure more time to allow his strategy to work, Bush -- in a televised prime-time address -- embraced recommendations by his top commander in Iraq for a limited withdrawal of about 20,000 troops by July.

But Bush also made clear his view that the United States would require a major involvement in Iraq for years to come and said the Baghdad government needed "an enduring relationship with America."

That assessment will make Bush's speech an even tougher sell with anti-war Democrats in control of Congress and with the large majority of Americans opposed to his Iraq policy.

"Because of the measure of success we are seeing in Iraq, we can begin seeing troops come home," Bush said after Gen. David Petraeus delivered two days of congressional testimony that underscored deep partisan divisions over the war.

Speaking in a sober, measured tone, Bush acknowledged Americans' frustration with the war but insisted progress was being made. His 18-minute speech was the centerpiece of a public relations offensive aimed at blunting demands for a faster, wider withdrawal from Iraq.

The partial drawdown will roll back troop strength from the current 169,000 to around the same levels the United States had in Iraq before Bush ordered a major buildup in January.

That prompted Bush's Democratic critics to accuse the administration of trying to fool the American people into thinking he was responding to growing anti-war sentiment when he was actually making no fundamental change in approach.

House speaker Nancy Pelosi said Bush had announced "a stay-the-course strategy that puts us on a path for 10 years of war," and Democratic National Committee chairman Howard Dean called it a "PR stunt to buy more time" for a failed policy.

ONE BRIGADE OUT BY CHRISTMAS

Bush said he had accepted Petraeus's proposal for the removal by mid-2008 of five of 20 U.S. military brigades now in Iraq, and that the pace of reductions would hinge on the level of success on the ground. He said 5,700 Marines and soldiers would be home by the end of the year.

U.S. officials refused to say exactly how many troops would be involved in the eventual withdrawal, though Petraeus had recommended that force levels return to where they stood before Bush boosted forces earlier this year.

An army brigade is typically made up of roughly 4,000 soldiers plus an unspecified number of support troops, which would make for a total withdrawal of more than 20,000 under Petraeus's plan. The so-called "surge" over the past eight months involved deployment of about 21,500 combat troops.

"The more successful we are, the more American troops can return home," Bush said. He said Petraeus would report to Congress again in March.

Bush cited Iraq's western Anbar province as evidence his strategy was making headway.

But underscoring the fragility of the situation, a Sunni tribal leader instrumental in battling al Qaeda in the area was assassinated on Thursday. Bush, who met Abdul Sattar Abu Risha during a visit to Anbar last week, praised his bravery in his speech.

Bush also acknowledged that the Iraqi government "has not met its own legislative benchmarks," and pressed it to do more to achieve national reconciliation.

He said U.S. engagement in Iraq would continue past the end of his term in January 2009, suggesting the job of ending the war would fall to his successors.

"This vision for a reduced American presence also has the support of Iraqi leaders from all communities. At the same time, they understand that their success will require U.S. political, economic, and security engagement that extends beyond my presidency," he said.

The drawdown would not be as fast or extensive as critics demand, but it could buy time for Bush to pursue the war by undermining a Democratic-led push for a broader disengagement 4-1/2 years after the invasion that toppled Saddam Hussein.

Some of Bush's fellow Republicans have also voiced doubts over his strategy. Republicans lost control of Congress in last November's election, largely due to public disenchantment over Iraq. Recent polls show Americans two-to-one against the war.

Democrats say the White House was putting the best political spin on what Pentagon officials have been saying for months -- that the buildup of forces in Iraq faces a time limit because of the risk of overstretching the U.S. military.

(Additional reporting by Caren Bohan, Deborah Charles and Kristin Roberts)

Iraq Nancy Pelosi George W. Bush