An Iraqi official Monday defended efforts to prevent sectarian violence in Baghdad, saying government security forces are doing all they can to keep people from being killed or driven from their homes in mixed neighborhoods of Shiites and Sunni Arabs.

In Anbar, the volatile province west of Baghdad, a U.S. Marine helicopter carrying 21 people made a hard landing, injuring 18 on board, the military said. It was the third U.S. military aircraft to go down in the province in two weeks, although the military has said none was hit by gunfire. Separately, three U.S. soldiers were killed by a roadside bomb in the capital.

Citing one example of government efforts against sectarian violence in Baghdad, Defense Ministry spokesman Mohammed al-Askari said Iraqi soldiers rushed to Ghazaliyah, a primarily Sunni area of west Baghdad, on Monday to free 23 Iraqis right after they were taken hostage at a checkpoint set up by suspected Shiite militiamen.

"We killed one terrorist and arrested four others," al-Askari said.

Attacks against Sunnis in Hurriyah, a mixed neighborhood of northwestern Baghdad, have raised new fears of an organized campaign by Shiite militants to drive Sunnis from the area and strengthen militia control of the capital's north. Witnesses say scores of Sunni families have been fleeing the Hurriyah neighborhood in recent weeks, and Sunni organizations claim that the Shiite-dominated Iraqi army and police have done little to stop the violence.

Al-Askari said five Iraqi army companies are stationed in Hurriyah to protect all its residents.

"Some people are using places such as Hurriyah to support their false claim that the government and its security forces are incapable or unwilling to stop such violence," he said.

Iraqi commanders said they are encouraging Sunnis to remain in Hurriyah and assuring them of their safety. Still, the authorities are clearly struggling to curb the violence.

Hurriyah was relatively calm Monday, two days after about 600 Iraqi soldiers were sent there in response to clashes in which police said at least two people were killed and two others wounded.

"We can't deny the presence of the outlaws in Hurriyah who have managed to intimidate residents and force some of them out of their houses. But Hurriyah isn't the only area where this is happening in Baghdad. It's going on in other neighborhoods, too, and all Iraqis are being targeted, not only one sect," al-Askari said.

Three explosions struck Baghdad within a span of two hours Monday, police said, after a roadside bomb in the capital killed three U.S. soldiers and wounded two on a late-night patrol. Insurgents often plant such explosives and hide nearby to set them off with hidden electrical cords or cell phone devices as coalition convoys pass.

The three deaths raised to 46 the number of American troops who have died this month. At least 2,934 members of the U.S. military have died since the war started in March 2003, according to an Associated Press count.

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The Marine helicopter made a hard landing on a routine flight in Anbar, the military said. Eighteen people were injured, including nine who were treated and returned to duty.

On Dec. 3, a Sea Knight helicopter carrying 16 Marines went down in a lake in Anbar, killing four. On Nov. 27, a U.S. Air Force fighter jet crashed in a field in Anbar, a province the size of North Carolina that is a base for many of Iraq's Sunni Arab insurgent groups.

At 9 a.m. Monday, a suicide car bomb hit an abandoned house being used by policemen as an outpost in Dora, southern Baghdad, killing one policeman and wounding five, a police officer said on condition of anonymity because he is not authorized to speak to the media.

At 9:45 a.m., a roadside bomb exploded near Mustasiriyah University in east Baghdad, wounding seven civilians standing nearby, said police Lt. Ali Muhsin.

A parked car bomb detonated at 10:30 a.m. near al-Maamoun college in western Baghdad, killing one student and wounding two others and two policemen, a police officer said on condition of anonymity out of concern for his safety.

Last week, Prime Minister Nouri al-Maliki urged university professors and students — who are often targeted by violence — to ignore a Sunni Arab insurgent group's warnings to avoid classes, calling them "desperate attempts." The group had sent e-mails to students and posted signs at schools and mosques in Baghdad, saying students should stay away while it cleanses the campuses of Shiite death squads, according to a statement from al-Maliki's office.

Elsewhere in the capital, three mortar rounds killed four people and wounded 15, and gunmen stole $1 million from a bank truck and kidnapped its four guards, police said.

In attacks outside Baghdad, three policemen and four civilians were gunned down, and funerals were held in a village near Kirkuk, 180 miles north of Baghdad, for a family of five Shiites, including a pregnant woman, who had been gunned down at their home Sunday.

Meanwhile, Iraq and Syria held ceremonies in each other's capitals to celebrate their decision last month to restore diplomatic relations. Syrian officials raised their flag at the Syrian Embassy in Baghdad, and Iraqi officials raised theirs at their embassy in Damascus.

Syria had broken diplomatic ties with Iraq in 1982, accusing it of inciting riots in Syria by the banned Muslim Brotherhood. Damascus also sided with Iran in the 1980-88 Iran-Iraq war.

Trade ties between Iraq and Syria were restored in 1997.