Homicide car bombers struck twice Sunday in the northern city of Mosul, killing at least six people and wounding dozens of others, U.S. and Iraqi officials said. A car bomb killed seven other people in Baghdad.

Two Iraqi soldiers were killed by snipers in separate attacks Sunday in the capital's Yarmouk district, police said.

Also Sunday, the government announced new security measures to protect Christians in Mosul after a spate of attacks against them by Sunni religious extremists.

The series of attacks shows the ongoing security challenges facing Iraq as the U.S. shifts responsibility to this country's own soldiers and police following the sharp decline in violence since last year.

The first attack in Mosul occurred when a suicide car bomber attacked a U.S. patrol, the U.S. military said. There were no American casualties, but five Iraqis were killed, including three young boys, the U.S. said. The attack also killed the bomber.

Another homicide car bomber targeted Iraqi police in Mosul, 225 miles northwest of Baghdad. Twenty-five people were wounded, the U.S. said.

In Baghdad, a parked car bomb exploded in a commercial street in the Bayaa district, killing seven people and wounding nine others, police said. They spoke on condition of anonymity because they were not supposed to release the information.

The southwestern Baghdad neighborhood was the scene of bitter Sunni-Shiite fighting until last year when the U.S. troop "surge," the Sunni revolt against Al Qaeda and a cease-fire by anti-American cleric Muqtada al-Sadr brought down violence to its lowest level in four years.

"Several car bombings have occurred on this street but no measures were taken to prevent these events," one Bayaa resident, who gave only his nickname Abu Ibrahim, told Associated Press Television News. "Where is the government? Where are the security officials to prevent such attacks?"

Attacks have been continuing in Mosul, Iraq's third largest city, despite months of U.S. and Iraqi security operations against Al Qaeda and other Sunni extremist groups.

The governor of the province that includes Mosul, Duraid Mohammed Kashmoula, said Saturday that about 3,000 Christians have fled the city over the past week alone to escape threats and attacks by Sunni extremists.

Interior Ministry spokesman Maj. Gen. Abdul Karim Khalaf said Sunday that the government was taking new measures to protect Mosul's Christians, including more police in their neighborhoods and more checkpoints and patrols near churches.

"Anyhow, there is a kind of exaggeration in describing the events in Mosul," Khalaf said. "We don't deny that hostile acts occurred, but we have the ability to stop such acts and the situation is under control."

In an interview Sunday with Al-Sharqiya television, Iraqi Cardinal Emmanuel III Delly condemned the killings that have occurred recently in Mosul "especially our sons the Christians."

"We the sons of Iraq, should be of one heart, one population and one homeland for the sake of the prosperity of our country," he said from Rome.

In Baghdad, Iraq's Sunni vice president, Tariq al-Hashemi, said the plight of Mosul Christians indicated "a real defect" in the security operation there and urged the government to take steps immediately to correct the problems.

He said Christians in the northern city "have the right to be protected from those criminals and murderers."

U.S. officials are concerned that violence may rise in the run-up to provincial elections, which are expected by the end of January. Voters will select ruling councils in most of the country's 18 provinces. No date for the election has been set.

On Sunday, the spokesman for the election commission, Qassim al-Aboudi, told reporters that a total of 440 provincial seats would be contested, with 57 of them in Baghdad.

About 20,000 people have been trained as election monitors, he said.