Baghdad ? Motorcycle bombs killed at least 20 people in separate attacks in Baghdad Friday, at least 19 of them 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.

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.