Police say at three explosions occurred within minutes of each other in the capital's Dora district, while another car bomb went off in a busy street in a central Baghdad

Iraqi officials say a series of car bomb explosions in Baghdad has killed at least 11 people and wounded more than 30 others.

Police say at three explosions occurred within minutes of each other in the capital's Dora district, while another car bomb went off in a busy street in a central Baghdad during the Thursday morning rush hour.

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There has been a surge in violence in Iraq this week. On Wednesday, at least 25 people were killed and more than 70 wounded in separate attacks in Baghdad's mainly Shi'ite Sadr City district and the oil rich northern city of Kirkuk. More than 100 people were killed in attacks around the capital Tuesday.

Meanwhile, the U.S. military said an unmanned American aircraft crashed near Baghdad's airport Wednesday, and there were no reports of injuries on the ground. The military said the crash does not appear to be from hostile activity.

In other news, a private U.S. aid organization said one of its American workers, a woman, and three private security guards were ambushed and killed in Baghdad Wednesday.

An official with the Washington-based National Democratic Institute said it is not clear who carried out the attack. The official said the slain security personnel were from Croatia, Hungary and Iraq.

The National Democratic Institute is a non-profit organization that is providing training and advice to Iraqi political parties, civil groups and members of parliament.