BAGHDAD (AFP) - The United Nations (UN) warned on Friday that Iraq is at a "crossroads" and appealed for restraint, as four days of violence, including several attacks on mosques, killed more than 200 people.

"I call on the conscience of all religious and political leaders not to let anger win over peace, and to use their wisdom, because the country is at a crossroads," UN envoy Martin Kobler said in a statement.

The call came a day after Prime Minister Nuri al-Maliki warned of a return to "sectarian civil war".

Mr Maliki, from Iraq's Shi'ite Muslim majority, called on people "to take the initiative, and not be silent about those who want to take the country back to sectarian civil war".

Clerics Abdulghafur al-Samarraie and Saleh al-Haidari, who respectively head the Sunni and Shiite religious endowments, have also warned against sectarian strife.

Sunni-Shi'ite sectarian violence in Iraq, including bombings and death squad murders that peaked in 2006 and 2007, claimed tens of thousands of lives.

On Friday, bombs exploded at three Sunni mosques in Baghdad and a fourth north of the capital, killing at least four people and wounding 50, an official and medics said.

More than a dozen people were killed in attacks at Sunni mosques on Tuesday.

And a motorcycle bomb in the Baghdad Shi'ite district of Sadr City killed at least five people and wounded 21, while a roadside bomb in Dura in the capital's south wounded at least four, officials said.

With tensions high, a Sunni cleric at the site of a long-running anti-government protest near Ramadi, west of Baghdad, called in a Friday sermon for the creation of an army to defend Sunnis.

Sheikh Hamed al-Kubaisi urged each Sunni tribe to provide 100 people.

An AFP journalist saw between 60 and 70 men who had responded to Kubaisi's call near the site of the protest, armed with Kalashnikov assault rifles and rocket-propelled grenade launchers.

"I joined... to defend Sunnis," said 24-year-old Omar al-Hadithi, one of the men. "I am not afraid of arrest. I am ready to fight anywhere."

Meanwhile, security forces moved back into the northern town of Sulaiman Bek after gunmen who seized it withdrew, according to local official Shalal Abdul Baban.

The gunmen pulled out of the predominantly Turkmen Sunni town in Salaheddin province under a deal worked out by tribal leaders and government officials, Baban and municipal council deputy chief Ahmed Aziz said.

The gunmen had swarmed into Sulaiman Bek on Wednesday after deadly clashes with security force, who pulled back as residents fled.

Mr Baban also said helicopter fire wounded six people on the roof of a house in the town early on Friday.

Army Staff General Ali Ghaidan Majeed told AFP on Thursday that the gunmen in Sulaiman Bek, who he said numbered about 175, had been given 48 hours to withdraw or face attack.

The seizure of the town came amid a surge of violence which began on Tuesday when security forces moved in against anti-government protesters near the northern Sunni Arab town of Hawijah, sparking clashes that left 53 people dead.

Dozens more were killed in subsequent unrest, much but not all of it apparently linked to Tuesday's clashes, bringing the death toll to more than 200 by Friday.

The violence is the deadliest so far linked to demonstrations that broke out in Sunni areas of the Shiite-majority country more than four months ago.

The Sunni protesters have called for the resignation of Maliki and railed against authorities for allegedly targeting their community, including with what they say are wrongful detentions and accusations of involvement in terrorism.

In other violence, seven gunmen died in attacks on security forces south of the northern city of Kirkuk on Friday, a high-ranking army officer and a medical source said.

And gunmen killed a Sahwa anti-Qaeda militiaman south of Kirkuk, while seven Katyusha rockets wounded two civilians in the city, police and a doctor said.

Gunmen also killed a soldier and wounded two in an attack on a checkpoint near Baiji, police and a medical source said.