At least 27 people were killed on Sunday in a series of suicide bombings, including one where an ambulance was used, which were claimed by Islamic State in Iraq's northern province of Salah al-Din.

The deadliest attack took place in the city of Tikrit, where an ambulance packed with explosives went off at a security checkpoint killing 15 people and injuring 35, a security official said.

The dead included a senior army officer and four policemen, he said.

Twelve people, including four Iranian pilgrims, were killed and 23 injured in two suicide car bombings in a public transport garage in the holy city of Samarra, the official said on condition of anonymity.

The authorities imposed curfews in Tikrit and Samarra for fear of further attacks.

Islamic State's semi-official media outlet, the Amaq agency, said that the radical Sunni militia was behind the attacks.

Two ‘martyrdom’ bombers attacked a crowd of Shia Iraqis and Iranians near a holy shrine in Samarra, Amaq said, using a jihadist term referring to suicide attackers.

A suicide car bomb meanwhile hit a security checkpoint in Tikrit, it said.

Islamic State regards Shias as heretics.

Sunday's attacks come as Iraqi forces, backed by a US-led air alliance, are pressing ahead with an offensive to dislodge Islamic State from its stronghold in Mosul, the country's second-largest city.

The extremist group has stepped up its attacks in different parts of Iraq since the Mosul campaign started on October 17.

Iraq has since reported advances in the mission to retake Mosul that has been under Islamic State control since mid-2014.

Iraqi forces were Sunday engaged in street fighting against Islamic State militants in a residential district on the northern edge of Mosul, army officer Walid Khaled told dpa.

He said that similar fighting was under way in two other neighbourhoods east of Mosul.

The officer did not give casualty figures.

Islamic State is believed to be holding thousands of civilians in and around Mosul as human shields.

In recent months, the al-Qaeda splinter group has suffered military setbacks and lost ground in Iraq and neighbouring Syria.