BAGHDAD — A parked car bomb exploded in a commercial district of central Baghdad today, killing 18 people and wounding dozens more.

The bombing took place off a bridge at Tahrir Square, a district of clothing shops just outside the heavily fortified Green Zone, which houses the American Embassy and much of the Iraqi government, a police official said on condition of anonymity because he was not authorized to release the information.

The policeman and a hospital official said 18 people died. The hospital official said 57 others were injured.

The attack is the latest in a string of violence to grip Iraq’s capital after several months of relative calm that followed a surge of American forces last year.

There also has been a sharp increase of American military deaths in recent days. Twelve Americans have been killed in the past four days, bringing the overall American military death toll since the start of the war to 3,987, according to an AP count.

The American military said today that soldiers had killed a young Iraqi girl after firing a warning shot at a woman who “appeared to be signaling to someone” along a road where several bombs had recently been found.

The shooting occurred yesterday afternoon in the volatile Diyala province north of Baghdad. An exact location was not given in a military statement.

The girl appeared to be “around 10 years old,” a military spokesman, Major Brad Leighton, said.

In its statement, the military said “coalition forces fired a warning shot into a berm near a suspicious woman who appeared to be signaling to someone while the soldiers were in the area. A young girl was found behind the berm suffering from a gunshot wound.”

Major Leighton said preliminary reports indicated that soldiers did not believe the woman was a potential suicide bomber, but rather “they were afraid she was signaling to someone that the convoy was going by.”

Also Thursday, the Chaldean Catholic Archbishop, Paulos Faraj Rahho, was found dead near the city of Mosul, where he had been kidnapped last month, an auxiliary bishop of Baghdad, Monsignor Shlemon Warduni, said.

Archbishop Rahho was kidnapped by gunmen soon after he left Mass in Mosul. Three of his companions were killed, the latest in what church members called a series of attacks against Iraq’s small Christian community.

The Chaldean church is an Eastern-rite denomination that recognizes the authority of the pope and is aligned with Rome.

The Vatican said Pope Benedict XVI was “deeply saddened” by Rahho’s death.

“We had all kept hoping and praying for his release,” a Vatican spokesman, the Rev. Federico Lombardi, said. “Unfortunately the most absurd and senseless violence keeps dogging the Iraqi people, and especially the small Christian community.”

In other violence, five members of an Awakening Council were killed when gunmen attacked two separate checkpoints near Tikrit on Thursday, 80 miles north of Baghdad. Nine others were wounded.

A suicide bomber also attacked an Awakening Council gathering at the village of Zab outside Kirkuk, 180 miles north of Baghdad. Three people were killed and seven others wounded in that attack.

Awakening Councils are made up of mostly Sunni fighters who have accepted American backing to switch allegiances and fight Al Qaeda in Iraq.

Unknown gunmen also killed a correspondent for a Baghdad newspaper. Qassim Abdul-Hussein al-Iqabi, 36, was shot while walking at Baghdad’s largely Shiite Karradah neighborhood, police said.

Excluding Mr. al-Iqabi, the New York-based Committee to Protect Journalists has recorded at least 127 journalists and 50 media support workers killed since the American-led war began in March 2003.