BAGHDAD, (Reuters) – A bomb killed at least 13 people and wounded 45 when it exploded in an industrial area of Baghdad on Friday, just four days before U.S. combat troops are due to withdraw from Iraqi cities and towns.

Police said the bomb was planted on a motorbike in a market specialising in motorcycles in Bab al-Sheikh, a largely Shi’ite Muslim part of central Baghdad.

Shredded shoes and bits of bloody clothing were scattered around the twisted frames of motorbikes.

A spate of bombings in Baghdad and other parts of Iraq has raised doubts about whether the Iraqi security forces can take over the fight against a stubborn insurgency when U.S. troops withdraw to their bases at the end of this month.

Iraqi police and army have had to be completely rebuilt since U.S. administrators disbanded the Iraqi forces after the U.S. invasion in 2003, a fatal decision that left thousands of trained fighters unemployed and angry, fuelling an insurgency.

Despite proud assertions from the government that the U.S. pullback respresents a victory for Iraq as it regains its sovereignty, many Iraqis lack faith in their own forces.

On Wednesday, 78 people were killed in a bomb attack in Baghdad’s Sadr City neighbourhood, just days after a truck bomb killed 73 people in the northern city of Kirkuk.

Such attacks have prompted angry responses from Iraqis who blame local security forces for failing to protect them.

Iraqi officials say the attacks are aimed at reigniting the sectarian warfare that raged for years between Iraq’s once dominant Sunni Muslims and majority Shi’ites, who have gained supremacy since the fall of Sunni dictator Saddam Hussein.

Violence is a far cry from what it was during the height of sectarian killing in 2006-2007, but Iraq’s untested forces and fractious political class still face major security challenges.

U.S. and Iraqi officials have warned they expect the number of attacks to rise as U.S. combat troops leave urban centres this month. Analysts say violence is also likely to spike in the run-up to a parliamentary election next January.

The U.S. withdrawal from urban combat posts by June 30 is a milestone in the plan to withdraw all combat troops from Iraq in August 2010 and pull out all U.S. soldiers by 2012.

More than six years after Saddam Hussein was ousted, and as the Obama administration looks increasingly towards the war in Afghanistan, the United States still has about 130,000 troops in Iraq.