The measure, introduced by a powerful Shiite group last month, creates a mechanism that many believe will lead to a predominantly Shiite zone in southern Iraq that would parallel the semiautonomous Kurdish region in the north. Sunnis vehemently oppose such a division, which would leave them with an area in central Iraq that lacks the vast oil wealth of the north and south.

Under a compromise worked out two weeks ago, the bill includes a provision that prevents the formation of federal regions for 18 months. In exchange for that delay and the creation of a panel to review the constitution, the Sunnis agreed to call off a boycott that had prevented the federalism bill from being introduced.

Although the principle of federalism is enshrined in the constitution, the law passed by parliament is the first to set up a system that will allow provinces in 2008 to merge into autonomous regions.

"This is a first step on a long road toward a new system of governance in Iraq," said Ridha Jawad Taqi, a legislator with the Supreme Council for the Islamic Revolution in Iraq, the Shiite political party that was the measure's strongest supporter. "It will be up to the residents of whatever province to submit a request and go through the system."

But opponents of the bill -- including not only Sunni Arabs but secular parties and the bloc of anti-American Shiite cleric Moqtada al-Sadr -- expressed fears that the federalism plan could increase sectarian tensions and push the country further toward civil war.

"This is the beginning of the plan to divide Iraq," said Adnan al-Dulaimi, leader of the National Accordance Front, a major Sunni bloc, according to the Associated Press. "We hope there won't be an increase in violence."

Nasaar al-Rubaie, a lawmaker from the Sadr bloc, said: "The present conditions are not conducive to establishing regions, because we lack a strong central government that can overrule the regions." In fact, he added, "the central authority is actually weakening instead of being solidified and strengthened."

Also on Wednesday, former electricity minister Aiham al-Sammarae was convicted of corruption and sentenced to two years in prison. Afterward, U.S. troops took custody of Sammarae, who holds dual Iraqi-American citizenship, because he feared being killed if he was placed in jail, said government spokesman Ali al-Dabbagh.

Dabbagh said that the Iraqi government had demanded his return and that the Americans agreed. "The government of Iraq expects that the Americans will respect Iraqi juridical authority," he said.

Lou Fintor, a spokesman for the U.S. Embassy, said he could not comment because "federal privacy law prohibits us from releasing information concerning U.S. citizens in the absence of a Privacy Act waiver."

The U.S. military on Wednesday also announced the deaths of four troops, bringing the death toll for American forces this month to at least 40. The military said a soldier was killed Wednesday after his patrol was struck by a bomb in central Baghdad, and three Marines assigned to the 1st Brigade of the 1st Armored Division were killed Monday in Anbar province.