A U.S. helicopter crashed in the Tigris river while searching for a missing soldier on Sunday, and the aircraft's two crew members were missing, the military said.

It did not say what caused the crash of the OH-58D Kiowa Warrior helicopter (search), attached to the 101st Airborne Division (search).

The helicopter was searching for a soldier missing when the boat he was in capsized earlier Sunday while on patrol. The other three soldiers in boat were safe, but two Iraqi police officers and an Iraqi translator were confirmed killed in the incident, said Maj. Josslyn Aberle, a spokeswoman for the 4th Infantry Division (search).

She said the search for the two pilots was underway. U.S. troops and Iraqi police sealed off the area and established checkpoints to secure the search and rescue operation.

U.S. troops rushing to the scene came under "limited and ineffective small arms fire," the spokeswoman said. An Iraqi policeman manning one of the checkpoints was killed in a drive-by shooting, witnesses said.

It was the fifth helicopter crash in Iraq this month -- three of them due to hostile fire.

U.S. troops arrested nearly 50 people Sunday in raids in the Sunni Triangle after attacks in the volatile region killed six American soldiers.

Most of the arrests occurred in Baqouba, 35 miles northeast of Baghdad, where 46 people were detained in a series of raids, the U.S. military said. Three were arrested for alleged anti-coalition activities and the rest for illegal weapons possession.

Soldiers of the 4th Infantry Division also seized 220 hand grenades in a raid on a house in the town of Mukayshifa, located south of Saddam Hussein's hometown Tikrit, according to spokeswoman Maj. Josslyn Aberle.

The raids in the Sunni heartland followed a series of bombings and attacks Saturday in which six soldiers were killed. One of them, from the 4th Infantry Division, died Sunday of wounds suffered when insurgents fired a rocket propelled grenade at his Bradley vehicle in Beiji on Saturday.

Five other U.S. soldiers were killed in two separate bombings Saturday in Khaldiyah and Fallujah, both located in the Euphrates River valley west of the capital. A blast Saturday in Samarra to the north of Baghdad narrowly missed an American convoy but killed four Iraqis and wounded about 40 others, including seven Americans.

A roadside bomb exploded Sunday near a U.S. patrol in Baghdad, but a U.S. soldier, speaking on condition of anonymity, said there were no U.S. casualties.

The latest deaths raised to 513 the number of U.S. service members who have died since the United States and its allies launched the Iraq war March 20. Most of the deaths have occurred in the insurgency by Saddam Hussein loyalists since President Bush declared an end to active combat May 1.

The Bush administration launched the war, claiming Saddam had violated U.N. resolutions requiring Iraq to destroy its weapons of mass destruction.

Nine months after the collapse of Saddam's regime, no such weapons have been found. On Sunday, David Kay, the former top U.S. weapons inspector in Iraq, said he believes Saddam had no weapons of mass destruction before the U.S.-led invasion. Kay said the challenge for the United States now is to figure out why intelligence indicated that the Iraqi president did have them.

"We led this search to find the truth, not to find the weapons," Kay said on the National Public Radio program "Weekend Edition." "The fact that we found so far the weapons do not exist, we've got to deal with that difference and understand why."

The Bush administration is now embroiled in a political dispute with the country's powerful Shiite Muslim clergy over the blueprint for returning sovereignty to the Iraqis by July 1. Grand Ayatollah Ali al-Husseini al-Sistani wants members of a new legislature chosen by the voters, rather than selected in regional caucuses as the United States plans.

U.S. officials say the continuing violence and the absence of an electoral roll or a census make it impossible to hold early elections. However, the United States cannot afford to offend the Shiite leadership, because Shiites are estimated to comprise about 60 percent of Iraq's 25 million people.

Muwafaq al-Rubaei, a Shiite member of the U.S.-installed Governing Council, told reporters Sunday following a meeting with al-Sistani that the ayatollah is sticking to his demand for elections and believes they can be held before July 1.

"The clerics' opinion is the opinion of the Iraqi people in general," al-Rubaei said. "The constitution shall be written by Iraqis elected by Iraqis and not by foreigners. Al-Sistani's call is still in place to hold elections."

U.N. Secretary-General Kofi Annan is expected to announce this week whether to send a team to Iraq to assess if early polls are possible. Washington hopes that the involvement of the United Nations will help break the deadlock and satisfy the Shiites.