A U.S. military helicopter crashed into the Euphrates River (search) on Wednesday, killing its two pilots. The craft was burning as it went down after an explosion, witnesses said, but the military said the cause of the crash was not known.

The U.S. military has lost 15 helicopters since the occupation began in May -- mostly to hostile fire. Sixty-two Americans have died in the crashes.

The OH-58 Kiowa (search) helicopter from the 3rd Armored Cavalry Regiment (search), based at Fort Carson, Colo., went down about 1:50 p.m. near Haditha, 120 miles northwest of the capital, and was lying on its side in the Euphrates, Brig. Gen. Mark Kimmitt said. He said it was not immediately known whether mechanical failure or enemy fire caused the crash.

In Haditha, Emad Rasheed, 45, said two U.S. helicopters were flying over the area when a missile hit one of them. Kimmitt said the second helicopter reported seeing no hostile fire.

Another witness, Saeed Hassan, 40, said the craft was on fire before it crashed.

"I was standing near the river bank when I heard an explosion and saw one of the helicopters fall into the river," said Ahmed Hamdan, a 35-year-old farmer. He and Hassan did not report seeing a missile.

The deaths brought to 547 the number of American service members who have died since President Bush launched the Iraq war on March 20. Most of the deaths have occurred after Bush declared an end to active combat May 1.

In action on the ground, U.S. soldiers captured five Iraqis after they attacked a convoy with a roadside bomb in Baqouba, the U.S. military said Wednesday. No one was injured.

The attack Tuesday morning caused little damage and was the latest in several attempts by guerrillas to strike against the coalition presence in the city, 35 miles northeast of Baghdad.

While violence against U.S. forces continues, Iraqi insurgents have been focusing more deadly attacks on Iraqi police and security forces working with the Americans. Gunmen assassinated a deputy police chief in the northern city of Mosul on Wednesday, and militants in the city of Kirkuk -- where a suicide bombing this week killed eight policemen -- littered police stations with leaflets warning of more attacks.

Meanwhile, in the Iraqi capital, two major ethnic groups -- Kurds and Turkomen -- pushed their causes as Iraqi officials struggle to draw up an interim constitution, a central pillar of U.S. plans for transferring power to an Iraqi government on June 30.

A Kurdish group presented a petition it said had 1.7 million signatures supporting a referendum on independence for Kurdish areas in northern Iraq. About 4,000 people held a rally demanding protections under the constitution for the Turkomen, who have expressed fears about Kurdish domination.

The constitution is due to be completed this week, but members of the U.S.-appointed Iraqi Governing Council are sharply divided over some of its most important points, including the makeup of the presidency and the shape of a Kurdish federal region.

In New York, the U.S. ambassador to the United Nations, John Negroponte, said the council may not meet the target date.

"The Iraqis have made significant progress toward the completion of the law and continue to work hard toward the achievement of the Feb. 28 deadline, although it is not certain that that deadline will be met exactly," he said Tuesday.

But the U.S.-led coalition in Iraq insisted the council would meet the deadline, set in the timetable for the power handover agreed on in November. Council members "have indicated to us that they think it will be complete by the time that they have indicated in the Nov. 15 agreement, Feb 28. They are moving forward," said coalition spokesman Dan Senor.

Some council members say a partial constitution may be agreed on by the end of the month, delaying key issues like federalism and the Kurdish question until a permanent constitution is drawn up next year.

Kurdish leaders, who oppose any postponement, are demanding terms of federalism that would give them extensive control of oil and other resources in their region and allow them to maintain a distinct Kurdish armed force.

The organizer of the petition, the Reform Movement of Kurdistan, said support was strong in Kurdish cities for going a step further -- full independence.

"We expect the majority to support independence," said Halkaut Abdullah, an official with the movement. "This is their ambition, although ambitions aren't necessarily realistic."

The Turkoman demonstrators demanded protections under the new constitution. "We think the new constitution should recognize the Turkomen as the third largest ethnicity in Iraq," said one of the organizers, Kanaan Shakir. "We are against any ethnic federalism. Iraq has been always a united country with one people."