Two U.S. Army UH-60 Black Hawk helicopters collided in midair Saturday and crashed into a residential neighborhood in the northern city of Mosul, killing at least 17 soldiers and injuring five others, the U.S. military said.

Military officials said one other soldier was unaccounted for. All of those on board were members of the 101st Airborne Division, based in Fort Campbell, Ky. The officials said the aircraft collided when one, attempting to dodge groundfire, climbed swiftly and hit the other.

Several witnesses contacted in Mosul also said the helicopters had collided, but one witness said at least one of the Black Hawks was hit by groundfire. The Reuters news agency quoted a U.S. officer at the scene as saying that a rocket-propelled grenade hit one of the helicopters. The crashes took place at about 6:15 p.m., when darkness had already fallen.

It was the deadliest single incident involving U.S. forces since the March 20 invasion of Iraq, surpassing the toll of a helicopter crash on Nov. 2. In that incident, a Chinook transport helicopter was shot down by a surface-to-air missile, killing 16 troops.

A U.S. military statement said that one helicopter was carrying members of a quick-reaction force and the other was transporting soldiers on a mission to northern Iraq. The cause of the incident is under investigation, the statement said.

U.S. troops and Iraqi police sealed off the crash scene in Mosul, 215 miles north of the capital. The helicopters reportedly went down near a major bridge crossing the Tigris River. Black Hawks are used as utility craft and can carry as many as 11 soldiers in addition to crew.

Helicopters are in wide use in Iraq both for transport and to carry out raids on suspected guerrilla hide-outs. Attack helicopters have come into even more frequent use recently during stepped-up assaults on resistance forces in Baghdad and elsewhere in central Iraq.

Three other U.S. helicopters have been shot down in the past three weeks. On Nov. 2, guerrillas in central Iraq fired a surface-to-air missile and shot down a Chinook transport helicopter, killing 16 U.S. troops; on Nov. 7, a rocket-propelled grenade struck a Black Hawk, killing six; and on Oct. 25, an RPG hit another Black Hawk, but there were no fatalities.

Mounting casualties in Iraq have added to pressures for the Bush administration to speed up plans to turn over authority to Iraqi leaders and technically end the occupation by next July. The White House hopes to reduce the U.S. troop presence in Iraq by next summer, but there are no proposals for U.S. troops to completely abandon Iraq, even when an Iraqi provisional government is in place.

Anti-U.S. violence has been on the rise in Mosul, a city that had been relatively peaceful in the months after the fall of Baghdad in April. At least four U.S. troops have been killed either in ambushes or when their vehicles struck roadside bombs this month in the city.

Iraqi civilians who support the occupation have also been targeted for assassination. On Saturday, gunmen shot and killed two people, an interpreter for the city government and his son.

By mid-evening in Mosul, there were no reports of casualties among Iraqi civilians where the U.S. helicopters went down. Ambulances and municipal firetrucks rushed to the scene, witnesses said.

"The Americans have closed off everything near the crash," said Bashar Darwish, a hotel employee in Mosul. Darwish said he saw one low-flying helicopter in flames before it hit the other. He said that there had been exchanges of fire between the choppers and someone on the ground, and that the Black Hawk on fire had erupted in flames after being hit.

He said the crash took place in the Sheik Fethi district on the city's west side. Another witness, Mohammed Badran, said one of the helicopters ascended abruptly and hit the second aircraft. Yezen Juburi, a businessman in Mosul, said that the helicopters simply collided.

The helicopters were attached to the 101st Airborne Division, which occupies Mosul. A spokesman for the unit declined to provide details about the crashes. A military official in Baghdad said that one of the Black Hawks was trying to avoid groundfire.

Elsewhere in Iraq on Saturday, a U.S. soldier was killed in Baghdad, the victim of a roadside bombing against one of two vehicles on patrol. Guerrillas have been using a variety of weapons to attack, including explosives, artillery shells, mortar shells and mines. The devices are sometimes set off when vehicles roll over them; sometimes they are detonated by remote control.

An Italian wounded in a car bombing last week in the southern city of Nasiriyah died Saturday in a Kuwaiti hospital; the Italian toll in the incident is now 19. The soldier's family gave medical personnel permission to take him off life support systems after he was declared brain dead.

The targets of car bombings in Iraq have included embassies, the U.N. headquarters in Iraq, offices of the International Committee of the Red Cross, police stations, and hotels. The Nasariyah attack was one of two in the relatively pacified southern part of the country. In late summer, a blast in the Shiite Muslim town of Najaf killed a religious leader and dozens of civilians.

Meanwhile, a kidnapped Portuguese journalist was freed unharmed Saturday in the vicinity of Basra, 36 hours after he was abducted. The journalist, Carlos Raleiras, told a Lisbon radio station that he was seized by nine gunmen, and was moved in the trunk of a car to several different houses during his ordeal.

U.S. troops secure an area where one soldier was killed and two wounded from a roadside bomb in Baghdad in an escalating campaign of violence.