Two explosions inside a Sunni mosque north of Baghdad, killed least 21 people, officials said. The bombs were hidden inside air conditioners, the same tactic used in a deadly bombing on a Sunni mosque in Baquba last Friday that killed 33 people.
Mizhar Fleih, deputy head of Samarra's municipal council, said the blasts went off around midday.
"During the Friday prayer suddenly a huge explosion took place," said 35-year-old Saleh al-Shamani. "Black smoke filled the mosque. We could not see one another. I tried to stand, but I couldn't because I had some injuries in my legs."
Fleih said that at least 21 people were wounded in the attack and that the Musaab bin Omair Mosque was heavily damaged.
"We are worried that the attacks on Sunni and Shia mosques aim at reigniting the sectarian strife in this country," he added.
It was not clear who was behind the attack, which took place a few miles south of of Samarra in al-Mafraq. Samarra is a largely Sunni city that is home to a revered Shia shrine. It is about 60 miles north of the Iraqi capital.
Iraq's delicate sectarian balance has come under growing strain from the civil war in neighboring Syria, where mainly Sunni rebels are fighting to overthrow President Bashar al-Assad, a leader backed by mostly Shia Iran.
Violence in Iraq has intensified since April to levels not seen since 2008. More than 4,000 people have been killed over the past five months, including more than 800 in August, according to figures provided by United Nations officials based in Iraq.
The bloodshed, 18 months after the withdrawal of US troops, has stirred concerns about a return to the sectarian violence of 2006 and '07, when the monthly death toll sometimes topped 3,000.
Al Jazeera and wire services