The attack targeted a Sunni Arab mosque in Tal Afar, a mostly Shiite Turkmen town in Nineveh province between the city of Mosul and the Syrian border.

It took place at around 12:30 p.m. local time on the Muslim holy day of prayer and left 67 people wounded, police and medical officials said.

Witnesses and security officials said that after the mosque's imam began to speak, the attacker pulled out a gun and shot him, then set off a belt full of explosives as other worshippers tried to tackle him.

"As the imam took his place and began to speak, someone in a black jacket pulled out a gun and killed him, shouting 'Allahu Akbar' (God is greatest)," said Mohammed Othman, who suffered chest and head injuries in the blast.

"When the people went after him, he blew himself up," the man in his 40s said, speaking from Tal Afar hospital.

Salah Ahmed, a 49-year-old worker who was sitting outside the mosque when he heard the gunshots, said he ran to "try and see what was going on, but then the explosion happened. I woke up to find myself in the hospital."

Hanni Mohammed at Tal Afar Hospital said the facility had received 12 dead bodies so far, and added that 67 people had been wounded, some of them taken to clinics close to the mosque.

"Some of the wounded have very serious injuries, and some of them are undergoing surgery," he said.

A local police officer confirmed the toll and added: "The bomber who blew himself up was among the worshippers inside the mosque as the imam was conducting Friday prayers.

He shot the imam with a gun, and then blew himself up."

Imam Abdel Satar Hassan was a member of the Iraqi Islamic Party, a Sunni grouping that did not immediately comment on the attack.

All roads leading to the mosque were closed by security officials after the attack, while new checkpoints were erected and stringent searches were being carried out, the police officer said.

Nineveh province has been hit by several major attacks against smaller towns as well as bombings in Mosul, Wardak, Sinjar and Khaznah in recent months that have killed more than 100 people.

Tal Afar was itself hit on July 9 by a double suicide attack targeting the home of a police sergeant and his brother, leaving 35 dead.

According to the Brussels-based think-tank International Crisis Group, insurgent groups remain active in Nineveh, although violence in Iraq has generally declined compared to previous years since the 2003 U.S.-led invasion.

In a report on the province last month, the ICG said the province's "continued strife risks dragging other parts of the country onto a downward slope."

Though attacks in Iraq as a whole have declined dramatically from last year, violence in and around Mosul and Baghdad remains common.

Violent deaths in Iraq dropped by more than half in September from the previous month, with 203 people killed. That was the lowest monthly toll since May.