Bombs targeting a take-out restaurant and a commuter bus in the Iraqi capital killed at least four people Monday, highlighting the challenges for Iraqi forces trying to maintain security in the cities after the withdrawal of U.S. combat troops.

The two bombings and another at a cafe in Baghdad on Sunday show that insurgents are trying to take advantage of the transition to Iraqi security forces that took place in the cities at the end of June to stage attacks that make residents feel vulnerable.

Iraqi officials are eager to dispel those fears and convince residents that Iraqi forces can continue the declining trend in violence over the past two years that has prompted many Iraqis to leave the safety of their homes and once again visit parks, restaurants and cafes.

A bomb explosion at the Shat al-Arab kebab shop in western Baghdad killed at least two people Monday, said police. The blast in the mixed Sunni-Shiite district of Bayaii also wounded four people.

A few hours later, a bomb attached to a minibus exploded during afternoon rush hour in the city's northern Shaab district, killing two people and wounding nine others, said police. The minibus was carrying commuters from the capital's central bus station.

A day earlier, a bomb attached to a motorcycle exploded near a popular cafe in a largely Sunni district of Baghdad, killing five people, authorities said. Sixteen people were wounded.

The bombings come as the U.S. military has begun winding down operations in advance of withdrawing combat troops by Aug. 31, 2010. A U.S.-Iraqi security pact requires all American troops to withdraw by the end of 2011. It also forced the U.S. withdrawal from Iraqi cities, leaving security in the hands of Iraqi forces.

Elsewhere Monday, gunmen killed a senior police official in Mosul, another police official said.

The officials all spoke on condition of anonymity because they were not authorized to release the information to the media.