Motorcycle bombs killed at least 20 people in separate attacks in Baghdad Friday, most in a crowded bazaar, part of an apparent trend toward increased use of motorcycles to thwart stepped up security measures.

The attacks were the latest in a week of violence that has killed more than 250 people, with just four days to go before the deadline for U.S. combat troops to withdraw from cities.

The spike has raised fresh doubts about the ability of Iraqi forces to provide security and fight a stubborn insurgency as their American partners become less visible.

U.S. and Iraqi officials have warned they expect more violence in the days surrounding the deadline as militants stage a show of force to try to stoke sectarian bloodshed and undermine confidence in the government.

But American commanders and Iraqi leaders have insisted the withdrawal will go ahead as scheduled. Under a security pact, the Americans must pull back from cities by June 30 and from the entire country by the end of 2011.

The deadliest blast occurred just after 9 a.m. when a booby-trapped motorcycle exploded in a market packed with young people buying or selling the vehicles in central Baghdad, according to police and hospital officials.

Ghaith Abdul-Allah, 35, was unloading motorcycles he planned to sell from his truck when the blast occurred.

"I saw a ball of fire and some motorbikes were lifted about 10 meters into the air," he said. "When the smoke from the explosion vanished, I saw a large number of young men lying on the ground soaked in blood."

"There were others who were screaming and crying for a lost brother or a friend. I do not know why these explosions are taking place and the Iraqi security forces are doing nothing to stop them," he added.

Nobody claimed responsibility for the attack, but insurgents frequently target crowded market districts to try to maximize casualties. The motorcycle bazaar is only open on Fridays, and police and hospital officials said the bomb on the motorcycle was loaded with nails and ball-bearings.

The market has been hit by several bombings in the past, but Iraqis had resumed flocking to the area due to a sharp drop in violence.

Attacks in Iraq have continued on a daily basis despite the security gains of the past two years, but many of the recent bombings have been larger in terms of numbers killed.

Police and hospital officials gave the death toll and said more than 50 people also were wounded.

Hours later, another explosives-laden motorcycle exploded in western Baghdad, killing at least one civilian and wounding three others.

The use of motorcycles underscores the resilience of militants as they adopt new tactics to penetrate the concrete walls and other measures aimed at preventing car bombs and homicide attacks.

An Iraqi Interior Ministry official said it was hard to pinpoint blame with motorcycle bombs as they have been used by Al Qaeda in Iraq and other Sunni insurgents as well as Shiite extremists.

"They have started to use motorcycles more because they are easy to move and can be used to avoid security checkpoints," the official said, speaking on condition of anonymity because he wasn't authorized to discuss sensitive information with the media.

The U.S. military and other Iraqi officials did not immediately respond to a request for comment.

The Associated Press has recorded five booby-trapped motorcycle bombings this month in Iraq that have killed at least 104 people, including one on June 24 in Baghdad's Sadr City that killed 78 civilians and injured another 143 — one of the deadliest bombings this year.

Before that, no parked motorcycle bombings had been reported in Baghdad since Aug. 19, 2007, when a motorcycle exploded in central Baghdad, killing one civilian and injuring four others.

Between January 25, 2007 and December 4, 2008, only five incidents involving motorcycle bombs were reported around Iraq, killing 25 people and wounding 110 more.

The numbers are based on AP reporting and are likely a minimum.

Followers of anti-U.S. Shiite cleric Muqtada al-Sadr, whose movement is gearing up to contest national elections on Jan. 30, blamed the Americans for the recent bombings, saying they were using them as an excuse not to withdraw completely.

Sadrist protesters took to the streets in Baghdad's Sadr City district and other cities after Friday prayers, burning American flags and denouncing the violence.

"The latest criminal acts inside Iraqi cities are clear proof that there will be no withdrawal and that the occupier is trying to find pretexts to stay in our holy land," al-Sadr aide Salman al-Fraiji said.