An Iraqi soldier was also killed at an checkpoint in Mosul on Wednesday.
In a separate incident, police opened fire on a car, killing a civilian. The reason for the shooting was not immediately clear.
North of the city, in another part of Nineveh province, two civilians were killed when their car exploded.
Police suspected they might have been transporting explosives.
Mosul, a volatile ethnic and religious mix of Arabs, Kurds, Christians and other minorities, has become Iraq's most violent city while the bloodshed that followed the US-led invasion in 2003 has declined in the rest of the country.
US combat troops withdrew from central Mosul, as they did from the centres of other towns and cities, at the end of last month as part of a plan to withdraw all US troops by 2012.