The measure, whose primary sponsors included presidential hopeful Sen. Joseph Biden, D-Del., calls for Iraq to be divided into federal regions for the country's Shiite, Sunni and Kurdish communities in a power-sharing agreement similar to Bosnia in the 1990s.
In his first comments since the measure passed Wednesday, al-Maliki strongly rejected the idea, echoing the earlier sentiments of his country's vice president.
"It is an Iraqi affair dealing with Iraqis," he told The Associated Press while on a return flight to Baghdad after appearing at the U.N. General Assembly in New York. "Iraqis are eager for Iraq's unity. ... Dividing Iraq is a problem and a decision like that would be a catastrophe."
Iraq's constitution lays down a federal system, allowing Shiites in the south and Kurds in the north to set up regions with considerable autonomous powers. But Iraq's turmoil has been fueled by the deep divisions among politicians over the details of how it should work, including the division of lucrative oil resources.
Many Shiite and Kurdish leaders are eager to implement the provisions. But the Sunni Arab minority fears being left in an impoverished central zone without resources. Others fear a sectarian split-up would harden the violent divisions among Iraq's fractious ethnic and religious groups.
On Thursday, Iraqi Vice President Adel Abdul-Mahdi said decisions about Iraq must remain in the hands of its citizens and the spokesman for the supporters of radical Shiite cleric Muqtada al-Sadr agreed.
"We demand the Iraqi government to stand against such project and to condemn it officially," Liwa Semeism told the AP. "Such a decision does not represent the aspirations of all Iraqi people and it is considered an interference in Iraq's internal affairs."
In other developments:
Iraq's prime minister also said he discussed the role of U.S. troops and private security contractors in the country, stressing that Iraq is a sovereign nation and it should have control over its own security.
Security "is something related to Iraq's sovereignty and its independence and it should not be violated," he said.
Al-Maliki's comments come after a Sept. 16 shooting in central Baghdad that killed some 11 Iraqi civilians allegedly at the hands of Blackwater USA guards providing security for American diplomats.
The North Carolina-based company said its employees were acting in self-defense against an attack by armed insurgents. Iraqi officials and witnesses have said the guards opened fire randomly, killing a woman and an infant along with nine other people, but details have widely diverged.
In a related story, a congressional investigation publicized Thursday found that Blackwater triggered a major battle in Iraq by sending an unprepared team of security guards into an insurgent stronghold, a move that led to their horrific deaths and a violent response by U.S. forces. Peter Singer, a senior fellow at the Brookings Institution, has been warning about the dangers of relying on private contractors since early in the war, reports CBS News national security correspondent David Martin.
The Washington Post reported Friday that a preliminary U.S. Embassy report found the shooting involved three Blackwater teams.
It said one was ambushed near a traffic circle and returned fire before fleeing the scene, another was surrounded by Iraqis when it went to the intersection and had to be extracted by the U.S. military and a third came under fire from eight to 10 people in multiple locations.
The report said the three teams had been trying to escort a senior U.S. official who had been visiting a "financial compound" back to the U.S.-protected Green Zone when a car bomb struck about 25 yards outside the entrance. The official was unharmed, it said.
An unnamed U.S. State Department official described the report to the newspaper and stressed it was only an initial account.
The New York Times also reported Friday that the shootings occurred as Blackwater was trying to evacuate senior American officials with the United States Agency for International Development after an explosion occurred near the guarded compound where they were meeting.
Participants in the operation said at least one guard continued firing on civilians while colleagues called for the shooting to stop, according to the newspaper's account, which cited American officials who have been briefed on the investigation.
It also said those involved have told U.S. investigators they believed they were firing in response to enemy gunfire but at least one guard also drew a weapon on a colleague who did not stop shooting.
American officials have publicly remained mum on their findings pending the results of a series of investigations.