Two U.S. military transport helicopters crashed Saturday night in southwestern Baghdad, the military said, killing two soldiers and bringing to six the number of American servicemen reported killed in violence across Iraq over the past two days.

Also in the capital, a series of bombings before dawn damaged five churches, and a mortar round or rocket struck a hospital, killing one Iraqi and wounding five.

Meanwhile, U.S. warplanes dropped bombs on the western part of the city of Fallujah, the center of a Sunni Muslim insurgency, and tanks fired artillery rounds from its perimeter, the Reuters news agency reported. The U.S. military did not report any civilian casualties, but residents said an infant was killed.

The U.S. helicopters crashed around 8:30 p.m., according to a statement released by the military, which said the cause was under investigation. Two wounded soldiers were evacuated and taken to a medical facility.

In one attack on U.S. troops, a suicide car bombing killed two soldiers, one Marine and an Iraqi translator Friday in Qaim, a city on the Syrian border in the province of Anbar that has become a center for Sunni insurgents. Another car bomb killed a U.S. soldier Friday in the northern city of Mosul, 220 miles north of Baghdad, the U.S. military said Saturday.

U.S. forces bombed Fallujah for 12 consecutive hours after two bombs exploded almost simultaneously inside Baghdad's fortified Green Zone on Thursday, killing three American civilians and as many as six Iraqis. Fallujah had been quieter for much of Saturday, before bombing resumed late in the day.

Troops formed a "dynamic cordon" around Fallujah to block "fleeing" insurgents, the military announced in a statement. The city has been bracing for a widely anticipated offensive by U.S. forces, who vacated Fallujah after a truce in April.

U.S. officials say that Abu Musab Zarqawi, a Jordanian linked to al Qaeda, uses the city as a base of operations. They denied a report by the official Kuwaiti news agency on Saturday that Zarqawi, who has asserted responsibility for numerous deadly attacks across Iraq, including the Green Zone bombing, had been captured during a raid.

Last week, insurgents and U.S. forces appeared to be drawing closer to a confrontation in Fallujah. On Wednesday, the interim prime minister, Ayad Allawi, warned Fallujah residents to hand over Zarqawi or face having the city taken by force. The insurgents responded Thursday with the Green Zone bombings, the first such attacks to penetrate the heavily protected compound. Negotiations for a cease-fire agreement remained stalled Friday as U.S. forces pounded the city with airstrikes and artillery.

U.S. officials have said intelligence indicates that insurgents intend to use the holy month of Ramadan, which began Friday, as a pretext for increasing attacks against U.S. and Iraqi forces throughout the country.

In Qaim on Saturday, one day after the attack on the Americans, mortar fire killed four Iraqis and wounded 30, a physician in the city told Reuters. Qaim is located near a highway that runs along the Euphrates River and connects to other Sunni hot spots such as Hit and Ramadi.

Homemade bombs exploded in quick succession early Saturday at the five churches in four Baghdad neighborhoods, the Associated Press reported. A series of similar bombings in August killed at least 11 people and injured dozens.

The Iraqi Interior Ministry said the first bomb exploded at St. Joseph Church at about 4 a.m. Over the next 90 minutes, bombs damaged St. Jacob's Church, St. George's Church, the Church of Rome and St. Thomas Church, the ministry said.

The Church of Rome, a Catholic church in the capital's Karrada district, was gutted, Reuters reported. Each church sustained exterior damage, according to news agencies.

"It is a criminal act to make Iraq unstable and to create religious difficulties," the Rev. Zaya Yousef of St. George's Church said, the AP reported. "But this will not happen because we all live together like brothers in this country through sadness and happiness."

Later in the day, a rocket or a mortar round struck the compound of the Ibn al-Bitar hospital in Baghdad. The attack killed one person and wounded five. A rocket also hit the parking lot of the Al-Mansour Hotel, where diplomats and journalists reside. No injuries were reported.

Iraqi women comfort each other outside St. Thomas Church in Baghdad's Mansour district, one of five churches damaged in pre-dawn bombings.