A suicide bomber exploded a minibus outside of a Shiite Muslim mosque south of Baghdad Wednesday, killing at least 20 people, while violence elsewhere in Iraq claimed the lives of four U.S. service members and more than a dozen Iraqi civilians and soldiers.

Two of the Americans died Wednesday morning when their helicopter reportedly was downed by a missile near Ramadi, an insurgent stronghold west of the capital. Hospital officials and residents of the area said that U.S. warplanes returned for a retaliatory strike later and dropped at least two bombs near the crash site, killing 20 people. A U.S. military spokesman in Baghdad said he had no information about the strike, which could not be independently confirmed.

The 5 p.m. blast outside the mosque in Musayyib, 35 miles south of Baghdad, came as worshipers were heading inside for sunset prayers to mark the end of their daily fast -- a central ritual of the holy month of Ramadan, which ends Thursday for Iraq's Sunni Muslims and Friday for Shiites. The Babil provincial police spokesman, Capt. Muthanna Ahmed, said a suicide bomber blew up a minibus in a square outside the mosque that was crowded with people from a neighboring bus station and grocery store. He said 20 people were killed and 64 were wounded.

The mosque was the site of a deadly explosion in July, when a suicide bomber detonated himself at a nearby gas station, blowing up a fuel tanker and killing at least 54 people.

Witnesses said the helicopter crash occurred during a clash between a U.S. convoy and insurgents from al Qaeda in Iraq, the radical Islamic group led by Abu Musab Zarqawi, a Jordanian. The incident occurred in Albu Obaid, a small town east of Ramadi, in the predominantly Sunni Arab province of Anbar, a stronghold of the insurgency.

"I saw the helicopter burning and falling down during clashes between a U.S. convoy and a group of Zarqawi's armed men," said Salah Otaiwi, 24, who lives in the neighborhood where the helicopter crashed.

Lt. Saad Hussein Dawood, a local police officer, said the helicopter was downed by a Strela missile, a shoulder-fired, heat-seeking projectile that was mass-produced by the Soviets in the late 1960s. He said it appeared that the helicopter was lured into the area.

"I think it was a clever ambush the Marines fell in," Dawood said. "The armed men shot at the convoy to make the helicopter hover over the area."

His account could not be independently confirmed.

The U.S. military identified the helicopter as an AH-1W Super Cobra, a two-person attack helicopter that can be armed with a wide variety of rockets and missiles. Two crew members from the 2nd Marine Aircraft Wing, 2nd Marine Expeditionary Force, died in the crash, according to a military statement.

The statement did not identify the cause of the crash, saying simply that it "occurred while the two-man crew was flying in support of security and stabilization operations." The cause of the crash is under investigation, the statement said.

Khalaf Abu Hussein, a local resident, said he and his family were still in bed when the helicopter crashed into their garden.

"We thought it was a rocket because the glass from the windows shattered everywhere," he said. "Three of my family members were wounded." Hussein said he did not see anyone inside the helicopter.

Later Wednesday, neighbors said, U.S. jets bombed two houses in the area. The attack killed 20 people and wounded 17, according to Khalid Dulaimi, a physician at Ramadi hospital, who claimed that U.S. military roadblocks in the area prevented rescue personnel from reaching the injured.

"The people gathered to rescue people whose house was bombed in the first strike," said Fadhil Khalil, 38, a neighbor. "A short time after they gathered, another plane came and bombed the house again, killing most of them." He said a third strike hit another nearby house, killing the entire family inside.

The military deaths brought to 126 the number of U.S. service members killed in helicopter crashes since the war began in March 2003, according to the Brookings Institution's Iraq Index.

A second military statement on Wednesday said that a U.S. Marine and a sailor were killed by a roadside bomb in Ramadi on Tuesday.

Elsewhere, five Iraqi civilians were killed and six were injured when the private minibus in which they were traveling was hit Wednesday morning by a roadside bomb in eastern Baghdad, according to Ali Mamoori, a spokesman for the Interior Ministry. The Associated Press quoted a police official as saying that the bus was traveling behind a U.S. military patrol.

In a separate incident, five Iraqi soldiers were killed and four were wounded when a roadside bomb exploded near their patrol in southern Baghdad, Mamoori said.

Iraq's Defense Ministry, meanwhile, called on junior officers who served in the Iraqi army during Saddam Hussein's rule to rejoin the armed forces. Officers who held the rank of major, captain or lieutenant before occupation authorities disbanded the Hussein-era military in May 2003 can apply for reinstatement this month, the ministry said in a statement.

The U.S. military and allied forces have been gradually training and equipping a new Iraqi military to fight an insurgency whose ranks are filled at least in part by former soldiers left jobless when L. Paul Bremer, Iraq's civil administrator, ordered the dissolution of Hussein's army.

Special correspondents Bassam Sebti in Baghdad and Saad Sarhan in Najaf contributed to this report.

Iraqis shovel debris from their charred shops after a mortar shell blasted a commercial street in Baqubah, northeast of Baghdad. No casualties were reported.A U.S. soldier patrols in Kirkuk, near the wreckage of a car that exploded before a passing convoy of American vehicles. One person was killed.