Baghdad: Gunmen shot dead five people, including the sister of an Iraqi policeman, in their beds in a house northeast of Baghdad early on Friday morning, a police official said.

The gunmen killed the policeman's sister, her husband's mother and father, their daughter and a child in their home in Hussainiya, northeast of the capital, the official said.

The policeman had called his sister, who did not answer the phone. He then went to the house and discovered she and the other four people had been killed, the official said.

The gunmen are suspected of having used silenced weapons as no one heard the shots.

It was unclear why the five were killed, with the official saying an investigation had been launched into whether it was a "criminal or terrorist" act.

According to Iraq Body Count, an independent Britain-based group, 4,021 civilians were killed in violence in Iraq from January 1 to December 30 last year, down from 4,680 in 2009.

"2010's civilian death toll in Iraq is the lowest since the war began" in 2003, IBC said in its report, while noting there was still an average of almost two explosions a day over the course of the year, with attacks occurring in 13 of Iraq's 18 governorates.

Prime Minister Nuri al-Maliki, who was approved for a second term by parliament along with a national unity cabinet on December 21, has named security as one of his top priorities.