The violence is among the most shocking in Iraq's worst prolonged period of unrest since it emerged in 2008 from a Sunni-Shia sectarian war, and comes with security forces also battling anti-government militants in western Anbar province.

The two incidents both took place in Salaheddin province, north of Baghdad, leaving nine dead in all, security and medical officials said on Sunday.

In Tuz Khurmatu, an ethnically-mixed town that has been hit by regular attacks, militants surrounded a police encampment protecting a stadium construction site and gathered the six policemen as a group and shot them all dead, two security officials and a doctor at the local hospital said.

One of the six, however, only died at hospital, and according to a local Tuz Khurmatu official, said that the militants had attempted to find out if the policemen were Sunni or Shia before killing them.

The militants asked them which sect they belonged to and the policemen, who were Shia Turkmen, initially said they were Sunni in an effort to save themselves, the town official said, speaking on condition of anonymity.

But because prayer rituals of Sunnis and Shias differ in certain key aspects, the victims were forced to pray as a group and their efforts to mask their confessional background were undone.

In a separate incident in Baiji, the decapitated heads of three men were found in a town market Sunday morning, two police officers in the town said.

The men -- an anti-Qaeda militia chief, his son and his cousin -- were kidnapped late on Thursday near provincial capital Tikrit.

The militiamen, known as the Sahwa, joined forces with the US military against their co-religionists in al-Qaeda from late-2006 onwards, helping turn the tide of Iraq's violent insurgency.

But, as a result, they are regarded as traitors by Sunni militants and often targeted in attacks.

Violence has surged in Iraq in recent months to levels not seen since 2008, with more than 1,000 people killed in January alone, according to government figures.

HH/HH