Seventeen American soldiers were killed and another five were wounded in Iraq after two U.S. Army UH-60 Black Hawk (search) helicopters crashed, the U.S. military said. One soldier remained unaccounted for.

The military said the cause of Saturday's crash was under investigation, but witnesses said one of the helicopters was hit by ground fire before it crashed into the other helicopter and a U.S. soldier said he heard reports that one of the choppers was hit by a rocket.

The spokesman for the 101st Airborne (search), Maj. Trey Cate, said the military was "trying to figure out what happened... We are going to do a thorough investigation because if this either involved ground fire or it was safety-related, then ... we're going to make sure we take precautions so it won't happen again."

The chief military spokesman in Baghdad, Col. William M. Darley, said the cause of the crash "will be under intense investigation today."

A statement by the U.S. command said one helicopter was carrying a quick reaction force and the other ferried soldiers on a transport mission in northern Iraq. Cate said the quick response team was on its way to investigate a shooting incident at a bank in which a U.S. soldier was injured.

The incident occurred around 6:30 p.m. local time in residential neighborhoods of Mosul (search), but both pilots were qualified for night flying, the military said.

The two helicopters belonged to the 101st Airborne Division from Fort Campbell, Ky., according to the military. The crash was the worst single loss of life for American forces since the beginning of the war in Iraq and pushed the U.S. death toll past the 400 mark. 

Also on Saturday, the Iraqi Governing Council (search) endorsed a U.S. plan that would create a provisional government by June. The transfer of power would provide Washington with an "exit strategy" in the face of escalating guerrilla warfare.

The two Black Hawks, which belonged to the 101st Airborne Division, went down in residential neighborhoods of Mosul, Iraq's third-largest city.

One soldier at the scene told The Associated Press he heard that one of the helicopters was hit by a rocket-propelled grenade before it crashed. A U.S. military spokesman said such reports were "at best speculative."

But an Iraqi policeman also said at least one of the Black Hawks was hit by ground fire.

"They hit it with a missile," said policeman Saddam Abdel Sattar. "I was in the army, I know these things."

Another witness, Yousra Khedr said she saw one helicopter above her house before hearing the sound of a loud boom.

"I saw the sky light up, it was like thunder and lightning," she said, adding that after the initial boom she heard gunfire in the area but could not say where it came from.

Before the crash, the U.S. military's deadliest incident was the downing of a Chinook transport helicopter on Nov. 2 that killed 16 soldiers. A Black Hawk was also shot down on Nov. 7, killing all six soldiers on board.

There were days early in the war in which more soldiers died, but they were spread over several attacks or accidents.

Earlier in the day, a 1st Armored Division soldier was killed by a roadside bomb in Baghdad. The crash put the number of American casualties since the March invasion at 417.

One witness, Nafe Younis, said he was sitting on the roof of his house when he saw the rotor blades of the two helicopters hit each other.

One of the helicopters then "hit into the house and a few minutes later it went ablaze," said Younis, who lives across the street from where one of the helicopters crashed.

As attacks against American occupation forces grow more sophisticated and deadly, Washington has become more concerned with handing power over to a new Iraqi government. The Bush administration has dropped its insistence that a constitution be drawn up and elections held before the transfer of power takes place.

Iraq's Governing Council, which has acted as Iraq's interim administration since it was appointed in July, on Saturday announced a set of deadlines that would give Iraq a provisional national assembly by May, a transitional administration with full sovereign powers in June and an elected government before the end of 2005.

With the return of sovereignty in June, the U.S. military occupation will formally end, although American forces are expected to remain in Iraq under a new arrangement to be worked out with the Iraqis.

Defense Secretary Donald Rumsfeld said U.S. troops will not withdraw anytime soon.

"The timetable or the way ahead that the Governing Council has been describing relates to the governance aspects of the country and not to the security aspects," Rumsfeld said in Japan. "That's on a separate track."

Until a constitution is drafted and adopted, a basic law will be drawn up by the Governing Council and take effect in February.

The law, according to an official statement, would establish a democratic and federal state that "respects the Islamic identity of the majority of the Iraqi people with the guarantee of the right of other religions and sects."

It will enshrine respect for human rights and ensure equality of members of the country's diverse religious and ethnic groups.

The new timetable replaced a political blueprint by L. Paul Bremer, the top U.S. official in Iraq, that envisaged a new constitution and a democratic government for Iraq before the end of 2004. The plan fell apart when council members could not agree on how to proceed with drafting a constitution.

The new timetable represented a victory for Iraqi politicians who have been lobbying strongly for a quick transfer of power.

"It is a great day in the history of Iraq," said Entifadh Qanbar, spokesman for council member Ahmad Chalabi. "We always called for restoring Iraq's sovereignty and this has become possible by this plan which was agreed upon by the Governing Council and our American friends."

Qanbar said the accelerated plan will be an important step in ridding the country of Saddam Hussein loyalists believed behind many of the attacks on U.S. troops.

However, Mahmoud Othman, one of five Kurds on the council, warned that implementing the timetable could prove difficult because of the security situation and acute unemployment, estimated between 60 and 70 percent.

"In my opinion things are going well, but if Iraq continues to lack security and the unemployment issue is not solved, then the implementation of the timetable won't be easy," Othman told The Associated Press. "Agreeing on paper is one thing and the implementation is another thing."

The council announcement followed Bremer's return from Washington on Thursday after two days of urgent consultations with President Bush and his top foreign policy advisers.

In Washington, Bush welcomed the new timetable as "an important step," saying it was essential to bringing democracy to Iraq and ensuring peace with its neighbors.

"The U.S. stands ready to help the governing council and all Iraqis translate this new timeline into political reality," he said in a statement.

The most difficult part of the plan may be the selection of the provisional assembly.

Chalabi, a moderate Shiite with close Pentagon links, said coordinating committees will be set up in each of Iraq's 18 provinces. The committees will choose nominees for the assembly. The nominees will in turn select assembly members from among themselves.

"This is a process which is as close as we can get to an elected body," said Chalabi. Bremer's coalition authority will not interfere in the selection process, he added.

In other developments Saturday:

-- The number of Italian military personnel killed in Wednesday's homicide attack in the southern city of Nasiriyah reached 19 Saturday when a severely wounded soldier was pronounced dead in Kuwait. Two of the dead were Italian civilians.

-- Gunmen killed a translator working for Mosul's municipal administration together with his son.

-- A Portuguese journalist abducted by gunmen in southern Iraq was released Saturday. Carlos Raleiras, a journalist with Lisbon-based radio TSF, said he was set free at a roadside about 36 hours after being abducted near Basra. He told TSF he was not harmed.

The Associated Press contributed to this report.