Fighters loyal to radical Shi'ite cleric Muqtada al-Sadr roll into the city of Samarra, Iraq, Sunday.

They call themselves the "Peace Brigade".

Located north of Baghdad, their aim is to fend off any possible incursion by Sunni militants.

(SOUNDBITE) (Arabic) UNIDENTIFIED FIGHTER OF "PEACE BRIGADE", SAYING:

"We volunteered in the peace brigade in response to the call of Sayyid Muqtada al-Sadr to protect the holy sites of Muslims, Christians and other sects in Iraq. God willing, we will be up to the expectations to protect the holy sites and defend Iraq's unity."

For the past month, Iraqi troops have been battling a lightning advance by radical Sunni militants, led by a group formerly known as the Islamic State in Iraq and the Levant.

The violent conflict threatens to divide the country.

On Monday, at least three civilians were killed when a car bomb exploded in central Baghdad. Several more were injured.

A second car bomb in a Shi'ite neig