The White House says the formation of Iraq's new government presents an opportunity to review the need for U.S. troops but is cautioning against setting timetables for withdrawals.

Yet, Iraq's prime minister said his forces will be able to take over security in all Iraqi provinces within 18 months.

President George W. Bush and British Prime Minister Tony Blair are to meet Thursday and Friday, and the future of Iraq under the new government and the role of their countries' troops there are at the top of the agenda.

Like Bush, Blair has dropped sharply in the polls and is under pressure to make troop cutbacks. His trip to the United States follows a visit to Iraq, where he said coalition troops are in position to begin handing over control of some Iraqi provinces to local security forces.

"I do not believe that you're going to hear the president or the prime minister say we're going to be out in one year, two years, four years," White House press secretary Tony Snow said. "I don't think you're going to get any kind of specific prediction of troop withdrawals. I think you're going to get a restatement of the general principles under which coalition troops stay or go."

In Baghdad, the new prime minister, Nouri al-Maliki, said Iraqi forces are capable of taking control of security in all the country's provinces within 18 months, but they need more recruits, training and equipment. Snow said al-Maliki is "a very aggressive, take-charge kind of guy" with whom the United States can work to get Iraqi forces trained as quickly as possible.

"At this point, everybody's taking the measure of the new government," Snow said. "What is going to happen, I think it's very safe to say, is that increasingly American, British and other coalition forces are going to move into support roles as Iraqi forces themselves move into lead roles."

About 132,000 U.S. troops are in Iraq, but officials have said they would like that number reduced to about 100,000 by the end of the year. About 8,000 British troops are in Iraq.

Snow said U.S. troops are in Iraq at the invitation of the government. If al-Maliki "says he doesn't need us, we're not going to stick around," Snow said.

Bush said at a Tuesday news conference that the swearing in of Iraq's new government has opened a door for change. The new Iraqi leaders will assess the country's security needs and forces, then work with U.S. commanders, he said.

But Bush has been careful not to commit to a timetable for withdrawals, even though most Americans say in this congressional election year they want to see a reduction of U.S. forces.

The two will meet privately, then will hold a joint news conference at 7:30 p.m. local time. That is early enough to avoid interfering with popular network television viewing in the United States but is after bedtime in Britain, where Bush is unpopular and most viewers will not be awake to see Blair appear live with the American president.