TIKRIT, Iraq — 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.