TIKRIT, Iraq (AP) _ An Army Black Hawk helicopter crashed Friday _ apparently downed by a rocket-propelled grenade, officers said _ killing six U.S. soldiers in what has become the deadliest week for Americans in Iraq since major combat was declared over in May.

Two Americans also were killed in separate attacks Thursday and Friday in the northern city of Mosul, raising concerns that the insurgency was spreading north.

A total 32 U.S. soldiers have died in the first week of November.

It was not immediately clear whether the chopper was brought down by hostile fire or a mechanical failure, a spokeswoman said. But several officers who asked not to be identified said it was probably hit by a rocket-propelled grenade.

``Six soldiers were on board and all of them were killed,'' said Maj. Josslyn Aberle, a spokeswoman for the 4th Infantry Division based in Tikrit. The dead included the four-person crew _ members of the 101st Airborne Division _ and two soldiers from Department of the Army headquarters in the Pentagon, said Army spokesman Maj. Steve Stover in Washington.

In a setback for U.S. planning, the government of Turkey said Friday it won't be sending troops to Iraq to relieve U.S. forces, following opposition to such a deployment from Iraqi officials.

Turkey's parliament had voted last month to allow a contingent of troops to join the U.S.-led occupation of its neighbor to the southeast. American officials had pressed Turkey, the only majority Muslim nation in NATO, to approve sending troops. But the Iraqis protested, saying they don't want troops from neighboring nations on their soil.

The United States has been trying to convince nations to send troops to Iraq as American forces face mounting attack.

If the Black Hawk was struck by a grenade, it would be the third helicopter downed by hostile fire in two weeks. Insurgents with a heat-seeking shoulder-fired rocket shot down a Chinook transport helicopter Sunday, killing 16 people in the deadliest strike against U.S. forces since the war began March 20. An rocket-propelled grenade forced down a Black Hawk north of Baghdad on Oct. 25, wounding one soldier.

All Black Hawks flying in Iraq are required to carry self-protection systems, including a mechanism that dispenses metallic chaff and flares to decoy an approaching heat-seeking missile, said an Army spokesman, Maj. Gary Tallman.

Such a defensive system may be of little use, however, against an RPG fired from close range, Tallman said, though he underlined that he did not know the circumstances of Friday's crash.

Aberle said the command was reviewing security in and around Tikrit, the hometown of Saddam Hussein, including considering whethter to reinstate a curfew lifted for the Muslim holy month of Ramadan.

White smoke could be seen rising from the crash site on the east bank of the Tigris River as three other helicopters circled overhead. More helicopters could be seen hours later flying over a hilltop village on the west bank of the river.

Separately, guerrillas attacked a convoy with rocket-propelled grenades and small-arms fire Friday morning in the eastern part of Mosul, 250 miles north of Baghdad. The military said one U.S. soldier died and six others were wounded in the clash.

Three others were injured later in the day when a roadside bomb exploded near the Mosul Hotel, which is now used as a military barracks, the military said. A military statement released Friday said a soldier died the day before near Mosul when a homemade bomb exploded.

The two dead soldiers in the Mosul area were from the 101st Airborne Division, the military said.

``We are saddened by the loss of any life,'' White House press secretary Scott McClellan said, traveling with President Bush in North Carolina.

The spate of attacks in the past week in Mosul, Iraq's third-largest city, has raised concerns among U.S. military commanders that the insurgency is spreading into that region from its main stronghold in the so-called Sunni Triangle, to the west and north of Baghdad.

The city is close to the semiautonomous Kurdish areas that lie between it and the Turkish border.

In Baghdad, about 500 people marched Friday toward coalition headquarters to protest the arrest of 36 clerics in recent months. They chanted Islamic slogans including ``America's army will be wiped out,'' and ``America is the enemy of God,'' and carried a banner reading ``Prisons ... will never terrify us.''

At the heavily fortified compound, a three-member delegation went inside and presented their demand to coalition officials. After a 45-minute meeting, the three returned, saying they had been promised ``that something good would happen,'' according to one of them, Sheik Awad al-Haradan.

Near Karbala, 70 miles south of Baghdad, the Polish brigade serving as part of the U.S.-led coalition held a memorial service for Maj. Hieronim Kupczyk, who was killed in an ambush Thursday.

It was the first combat death for Poland, which has 2,400 soldiers in Iraq and is in charge of a large swath of south-central Iraq, where about 9,500 soldiers of several nations help maintain security.

The Pentagon, meanwhile, announced that one of the soldiers wounded in Sunday's downing of an Army Chinook helicopter died Thursday at a medical facility in Germany, raising the death toll to 16. Twenty-six others were injured.

In al-Assad, a desert base 155 miles northwest of Baghdad, hundreds of soldiers, some wearing ceremonial spurs and black regimental hats, held a ceremony to remember their comrades killed in the shootdown, the deadliest single attack against U.S. forces since the Iraq war began March 20.

Army officials said the helicopter's crew apparently had a last-second warning of an approaching missile and managed to launch flares designed to draw the heat-seeking missile away. The defensive measure did not work and the missile slammed into the right side of the helicopter's rear engine.

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Associated Press correspondents Slobodan Lekic and Bassem Mroue in Baghdad, Mariam Fam in Mosul and Katarina Kratovac in Karbala contributed to this report.