Iraqi police say a bomb exploded Monday inside a bus in central Baghdad, killing four people.

Police say the bus was heading for the mainly Shi'ite district of Karradah when the bomb went off. At least four people were wounded.

On Sunday, three car bombs exploded in mainly Shi'ite areas of Baghdad, killing at least 60 people. It was the worst violence since U.S. and Iraqi forces began a new security campaign in the city last week. Sunday's blasts also wounded more than 100 people.

Officials say most of the casualties were caused by two car bombs at an outdoor market in the New Baghdad district. A third car bomb exploded in Sadr City.

In another development, the U.S. military says two American soldiers died Saturday on patrol in Baghdad, and a Marine was killed in western al-Anbar province.

Some information for this report provided by AP, AFP and Reuters.