BAGHDAD, Iraq - Iraq's transitional prime minister called today for a speedy withdrawal of U.S. troops and the top U.S. commander here said he believed a "fairly substantial" pullout could begin next spring and summer.

Prime Minister Ibrahim Jaafari said at a joint news conference with Defense Secretary Donald H. Rumsfeld that the time has arrived to plan a coordinated transition from American to Iraqi military control throughout the country.

Asked how soon a U.S. withdrawal should happen, he said no exact timetable had been set. "But we confirm and we desire speed in that regard," he said, speaking through a translator. "And this fast pace has two aspects."

First, there must be a quickening of the pace of U.S. training of Iraqi security forces, and second there must be closely coordinated planning between the U.S.-led military coalition and the emerging Iraq government on a security transition, he said.

"We do not want to be surprised by a withdrawal that is not in connection with our Iraqi timing,"' he said.

Speaking earlier with U.S. reporters traveling with Rumsfeld, Gen. George Casey, the top American commander in Iraq, said he believed a U.S. troop withdrawal could begin by spring 2006 if progress continues on the political front and if the insurgency does not expand.

Rumsfeld had said earlier today that Iraqi leaders must take a more aggressive stance against what he called harmful interference from neighboring Syria and Iran.

Rumsfeld arrived unannounced in the Iraqi capital with a series of messages for the country's interim leaders, suggesting a heightened sense of urgency in the Bush administration to make faster strides on the political and security fronts so U.S. forces eventually can leave.

He said he would be pushing the Iraqis to provide more people who can be trained by U.S. personnel to handle the growing number of detainees in the country, now estimated to number at least 15,000.

With a permanent Iraqi government scheduled to take power in January, following adoption of a constitution and an election in December, they need trained prison guards "so that as soon as it is feasible we can transfer responsibility for Iraqi prisoners to the Iraqi government," he said.

Rumsfeld has often criticized Iran and Syria for meddling in Iraq's affairs. In his remarks today, he put the main onus on Iraqi leaders to do more to fix the problem.

"They need to be aggressively communicating with their neighbors to see that foreign terrorists stop coming across those borders and that their neighbors do not harbor insurgents and finance insurgents," he said in an in-flight interview with reporters accompanying him from Tajikistan.

Rumsfeld singled out Syria and Iraq as troublemakers for the fledging government in Baghdad. The future course of their relations will help determine when Iraq will be stable enough to fight the insurgency on its own, he said.

"Their efforts to destabilize the situation in Iraq: Does it increase or decrease?" he asked.

That is among the many unknowns that make it impossible to predict when Iraq's own security forces will be strong and reliable enough to allow American troops to begin leaving, Rumsfeld said.

He also said he would urge the Iraqis not to miss their Aug. 15 deadline for completing the draft of a constitution to be submitted for a national vote in October in advance of subsequent elections for a new, permanent government.