The policeman was at work when unknown gunmen entered the family's home and fired a single shot into the head of each victim, said Colonel Sherzad Mofri, who listed the children's ages as three, six and nine.

"They killed his wife and three children who were sleeping beside their mother," said Mofri, announcing that an investigation was underway into what he said appeared to be a cold-blooded execution.

There were no signs of resistance from the victims, Mofri said, adding that the policeman whose family was wiped out by the attack had been convulsed by shock and so far been unable to be interviewed.

Mofri said he had never previously seen or heard of such a brutal, targeted attack being carried out on a family in Kirkuk.

The killings happened in the city's Banjar neighbourhood, an area where Kurds deported by Saddam Hussein, to win favour with Sunni Arabs, returned and built homes following the dictator's overthrow in the US-led invasion of 2003.

Kirkuk has a mixed population of Arabs, Kurds and Turkmen, and longstanding Kurdish demands for the city to be incorporated in their autonomous region in the north have fanned ethnic tensions.

In other violence in the region on Sunday, four Iraqi soldiers, one of them an officer, were killed when insurgents attacked their patrol 35 kilometres (20 miles) south of Kirkuk.

Two insurgents were also killed in the clash, police said.

Earlier on Sunday, an Iraqi army officer was killed in Kirkuk when a homemade bomb struck his patrol in the Rahimawa neighbourhood, a security official told AFP.

And in the restive city of Mosul, also in northern Iraq, four people were killed and one was wounded in what police said was inter-tribal score-settling following the death of another tribe member.

strs/adm/hc