A suicide bomber detonated a minibus Wednesday in an outdoor market packed with shoppers ahead of a Muslim festival, killing about 20 people and wounding more than 60 in a Shiite town south of Baghdad. Six U.S. troops were killed, two in a helicopter crash west of the capital.

Also Wednesday, the U.S. command confirmed moves to step up training on how to combat roadside bombs — now the biggest killers of American troops in Iraq. At least 2,035 U.S. military service members have died since the Iraq conflict began in March 2003, according to an Associated Press count.

The suicide bombing occurred about 5 p.m. in the center of Musayyib, a Euphrates River town 40 miles from Baghdad. On July 16, nearly 100 people died in a homicide bombing in front of a Shiite mosque in Musayyib (search).

Witnesses said the latest attack took place as the market was crowded in advance of the three-day Eid al-Fitr holiday that marks the end of the Islamic holy month of Ramadan (search). Many women and children were feared among the dead and wounded.

"They want to kill people before the feast," said Nagat Hassoun, 50, who lived a few hundred yards from the blast site. "They want people to stay at home and live in a tragedy. The aim is to cause sabotage. They're targeting the Shiites."

The town police chief, Lt. Col. Ahmed Mijwil, said 22 people were killed and 61 wounded. But officials warned the figures could change as rescuers frantically searched the area of meat and vegetable stalls, shops and cafes.

"The insurgents wanted to cause as many casualties as possible," said police Capt. Muthanna Khalid.

Elsewhere, fighting flared between U.S. troops and Sunni Arab (search) insurgents in Ramadi, capital of Anbar province 70 miles west of Baghdad. Late Tuesday, a U.S. Marine and sailor were killed when a roadside bomb exploded near their vehicle in Ramadi.

Sporadic clashes occurred throughout the night and into Wednesday, residents said. Associated Press Television News video from the city showed a burning civilian vehicle and what appeared to be a destroyed U.S. Humvee (search).

A crowd of Iraqis gathered at the site, and one man, waving the remnants of a damaged U.S. M-16 rifle in the air, claimed the attacks caused U.S. casualties.

Later Wednesday, a Marine AH-1W Super Cobra (search) attack helicopter crashed just north of Ramadi, killing its two Marine crew members, the military said. A U.S. statement said the cause of the crash was under investigation.

But APTN quoted an Iraqi resident as saying the helicopter was shot down. Hours after the crash, a Marine Corps F-18D fighter jet dropped two 500-pound bombs on what the U.S. military described as an "insurgent command center" about 400 yards from where the helicopter went down.

There was no report of casualties in the airstrike.

In Balad, 50 miles north of Baghdad, a U.S. soldier was mortally wounded when his patrol came under small arms fire Wednesday, the military said. One insurgent was killed when the American patrol returned fire and another died when a U.S. Air Force jet blasted the building where he had taken refuge, the military added.

The sixth fatality was a soldier from the Army's Task Force Baghdad who was killed by a roadside bomb Wednesday in a southern district of the capital, the military said.

The latest deaths follow the fourth deadliest month for American troops since the U.S.-led invasion of Iraq. Most of the 96 Americans killed in October were victims of roadside bombs.

Most of the insurgents are Sunni Arabs, and in an overture to that community Iraq's defense minister Wednesday invited some officers in Saddam Hussein's former army to enlist in the new force. The U.S. decision to disband Saddam's 400,000-member army soon after he was ousted in April 2003 has been widely seen as a major contributor to the growth of the insurgency, which is fueled by Sunni ex-soldiers.

The U.S. command said it is accelerating counterinsurgency training for newly arrived officers, including the best ways to protect their troops against roadside bombs.

Lt. Col. Steve Boylan, the U.S. military spokesman in Baghdad, said the U.S. command will soon open the training school at Taji, an air base 12 miles north of Baghdad. U.S. troops undergo counterinsurgency training before heading to Iraq, but the command wants newcomers to become familiar with the latest insurgent tactics.

There was no claim of responsibility for the Musayyib attack — the third major vehicle bombing in a predominantly Shiite area of Iraq within the last five days. A total of 50 people had already been killed since Saturday in car bombings in Basra and a Shiite village in central Diyala province.

Those attacks have fueled fears of more sectarian tension between the majority Shiites and minority Sunni Arabs.

The leader of Al Qaeda in Iraq, Abu Musab al-Zarqawi (search), has urged his followers to attack Shiites, whom he considers heretics and collaborators with the U.S.-led coalition.

The bloodshed persists despite progress in promoting a political process, which the Bush administration hopes will in time lure Sunni Arabs away from the insurgency and enable the Americans and their coalition partners to begin sending their troops home.

After voters overwhelmingly approved the constitution in an Oct. 15 referendum, attention has focused on the Dec. 15 parliamentary elections. Many Sunni Arab candidates have joined the race; most of their community boycotted the last election in January.

In other violence Wednesday, at least 13 Iraqis were killed and 25 wounded in scattered shootings and other bombings around the country.