Bus passengers kidnapped
The attack came after US troops withdrew from urban centres in June in line with a security pact between Baghdad and Washington calling for US forces to leave Iraq by the end of 2011.
Attacks increased in the run-up to the pullback, with 437 Iraqis killed in June - the highest death toll in 11 months. But attacks against security forces are still common in the capital.
Meanwhile, at least 11 Iraqi passengers were kidnapped when their bus was seized on the road from Baghdad to the southern city of Hilla, 40km to the south of Baghdad, police sources told the German Press Agency, dpa.
The source said that security forces were searching the area for the kidnappers.