TIKRIT, Iraq - Two bombs concealed in air conditioning units inside a Sunni mosque south of the Iraqi city of Samarra exploded during Friday prayers, killing at least 15 people, police and hospital sources said.

It was not clear who was behind the attack, which took place in al-Mafraq, around 10 km (6 miles) south of Samarra.

"During the Friday prayer suddenly a huge explosion took place. Black smoke filled the mosque, we could not see each other. I tried to stand, but I couldn't as I had some injuries in my legs," said 35-year-old Saleh al-Shamani.

At least 17 people were wounded, the sources said.

Iraq's delicate sectarian balance has come under growing strain from the civil war in neighbouring Syria, where mainly Sunni rebels are fighting to overthrow a leader backed by Shi'ite Iran.

Both Sunnis and Shi'ites have crossed into Syria from Iraq to fight on opposite sides of the conflict.

Al Qaeda's Syrian and Iraqi branches merged earlier this year to form the Islamic State of Iraq and the Levant, which has claimed responsibility for attacks on both sides of the border.

Iraq has also witnessed several incidents in recent weeks suggesting that Shi'ite militias, which have so far refrained from retaliating to attacks by Sunni insurgents, may once again be resorting to violence.

Around 800 Iraqis were killed in acts of violence in August, according to the United Nations.

Intensifying violence has raised concerns of a return to wider conflict in a country where Shi'ite Muslims, Sunnis and ethnic Kurds have yet to find a stable way of sharing power.