WASHINGTON - President Bush and Iraq's prime minister have agreed to set a "time horizon" for the withdrawal of U.S. combat troops from Iraq as part of a long-term security accord they are trying to negotiate by the end of the month, White House officials said Friday.

The decision, reached during a videoconference Thursday between Bush and Prime Minister Nouri al-Maliki, marks the culmination of a gradual but significant shift for the president, who has adamantly fought -- and even ridiculed -- efforts by congressional Democrats to impose what he described as artificial timetables for withdrawing U.S. forces.

In recent weeks, Bush and senior officials have hinted that they would be open to "aspirational" goals for removing U.S. troops, as al-Maliki and other Iraqi politicians have voiced increasing discontent with the idea of an open-ended U.S. troop presence in their country.

The White House has also been under pressure from top military officers to make more U.S. forces available for the war in Afghanistan, and that would be possible only by reducing the number of troops in Iraq, administration officials said. U.S. troop levels there have been decreasing in recent months, as they return to the 15 combat brigades present before Bush ordered a troop increase last year.

Senior military officials have made clear that they expect troop levels in Iraq to drop even further this fall, following a 45-day period of assessment by Gen. David H. Petraeus, the top U.S. commander in Iraq. In a statement issued Friday, after the conversation between Bush and al-Maliki, the White House went further than it has in previous official statements to indicate that it shares that expectation.

"In the area of security cooperation, the president and the prime minister agreed that improving conditions should allow for the agreements now under negotiation to include a general time horizon for meeting aspirational goals," the statement said. It said those goals include turning over more control to Iraqi security forces and "the further reduction of U.S. combat forces from Iraq."

A big change or not?

Aides to Bush portrayed the announcement Friday as consistent with the president's long-standing position that troop levels could be reduced in Iraq only as security conditions improved and as Iraqi forces showed greater capacity.

"I think it's important to remember that the discussions about timeline issues previously were from Democrats in Congress who wanted to arbitrarily retreat from Iraq without consideration of conditions on the ground," spokesman Scott Stanzel told reporters traveling with Bush on a GOP fundraising trip in Arizona and Texas.

Democrats seized on the White House statement as an about-face and an acknowledgment by Bush of the political realities at home.

Sen. Joseph R. Biden Jr., D-Del., chairman of the Senate Foreign Relations Committee, said Bush "has reversed course and dropped his adamant opposition to a timeline for redeployment of American troops from Iraq." He said the president "also has acknowledged the need to transition from a combat mission to one that focuses on training and counterterrorism." The administration, Biden added, "is finally facing reality."

Still more talking to do

The White House statement Friday was an effort to demonstrate unity between the U.S. and Iraq after recent reports that the two sides were having difficulty completing agreements that will govern long-term ties and rules for how U.S. troops will operate in Iraq.

Instead of the formal status-of-forces agreement that the administration had hoped to complete by the end of this month, the two governments are now working on a more limited "bridge" document that would allow basic U.S. military operations to continue into next year, U.S. officials said, after a U.N. mandate expires at the end of 2008.

Iraqi government spokesman Ali al-Dabbagh confirmed in a statement that Iraq and the United States had agreed "to specify a time horizon to achieve a full handover of security responsibility to the Iraqi forces in order to decrease American forces and allow for its withdrawal from Iraq."

But Sadiq Rikabi, a senior political adviser to al-Maliki, said in an interview that negotiators were still hashing out the details of troop cuts. The Iraqi government, he said, wants specific timelines governing different stages of what will eventually become a full U.S. withdrawal of combat forces.