BAGHDAD, Iraq — 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 suicide bombing in front of a Shiite mosque in Musayyib.

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. 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 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.

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 attack helicopter crashed just north of Ramadi, killing its two Marine crew members, the military said.