Two bombs tore through worshipers gathering for prayers at a Sunni mosque in Iraq on Friday, killing 18 people, while three died in other attacks, officials said.
Militants have carried out numerous attacks on both Sunni and Shiite mosques this year, raising fears of a return to the all-out sectarian conflict that peaked in 2006-2007 and killed thousands of people.
The bombs, which hit the Musab bin Omair mosque near Samarra, north of Baghdad, also wounded 21 people, police and a doctor said.
"A huge explosion happened, and everything around us was thrown in the air," Zaid Abdulwahid, a 26-year-old who was wounded in the blasts, told AFP at the main hospital in Samarra.
Questions for Abdulwahid had to be shouted, as his hearing was damaged by the explosions.
"We never expected that the mosque would be targeted," said Othman Ahmed, 23, another of the wounded.
He said he initially could not see anything after the blasts, but then noticed he was surrounded by dead and wounded people.
The February 2006 bombing of a Shiite shrine in Samarra itself sparked the wave of brutal sectarian violence that plagued Iraq in past years.
The blasts came a day after the bodies of 10 young men who had been shot dead were found in Baghdad, another reminder of the sectarian conflict, during which militants frequently carried out summary executions.
Also on Friday, mortar rounds killed a young girl west of Samarra and wounded two members of her family.
And gunmen kidnapped and killed a soldier in the northern province of Kirkuk, while a roadside bomb north of the city of Baquba killed one person and wounded three.
The United Nation's deputy special representative for Iraq, Gyorgy Busztin, expressed "extreme concern" this week about sectarian-based displacement of Sunnis and members of the small Shabak minority, and the killing of Sunnis in the country's south.
"The use of violence and intimidation against communities by illegal armed groups forcing them to flee their homes is unacceptable and a clear violation of basic human rights," Busztin said.
Prime Minister Nuri al-Maliki, who heads Iraq's Shiite-majority government, condemned the trend as he met with displaced Sunni families this week.
He said members of the Sunni Al-Saadun clan had been driven out of Nasiriyah, Basra and Kut, all Shiite-majority areas.
A Western diplomat said "ethnic cleansing is taking place each day. There are more and more attacks aimed at “widening the rift" between Iraqi communities.
In the port city of Basra, gunmen have shot dead 15 Sunnis since the start of the month, including a cleric and mosque employees, according to Abdulkarim al-Khazraji, a representative of the local Sunni community.
"Messages have been left in envelopes at night near Sunni mosques telling Sunnis to leave Basra" or face revenge attacks for violence against Shiites in the northern city of Mosul.
"An eye for an eye, a tooth for a tooth," the messages warned, according to Khazraji, a religious leader.
He said some families had fled, despite assurances of protection by the authorities, and all prayer services in Sunni mosques had been suspended.
Violence in Iraq has reached a level not seen since 2008, killing more than 4,200 people since the beginning of the year, according to an AFP toll based on security and medical sources.