Shia fighters from Iraq and Lebanon have travelled to Syria to help local allies defend a shrine, south of Damascus, which they believe is under threat from Sunni rebels, Reuters reports.
According to the news agency, both sources in Iraq and Syria and videos released by the fighters have confirmed the presence of combatants from neighboring states fighting with the Shia brigade.
Abu al-Fadl al-Abbas brigade, named after a seventh century martyr son of Imam Ali, considered the father of Shia Islam, was reportedly formed several months ago and fights mainly around the shrine of Sayyida Zeinab. 
An unnamed source told Reuters the Shias - a minority in Syria - set up the brigade to defend the shrine and mosque from Sunni fighters,  who have desecrated other places of Shia worship during the conflict.
"They are there for one purpose and that is to defend the shrine," he said, adding that they are operating independently of Assad's forces around the capital.
Two months ago, a video posted online showed Sunni Muslim rebels burning a husseiniya - a Shia religious site - in the northern province of Idlib. 
Rebels "wanted to destroy the Sayyida Zeinab shrine and hundreds of Iraqi Shias who were already living in Syria stood up to them and fought back," a source in Iraq told Reuters.
"Now they are more organized, under the Abu al-Fadl al-Abbas Brigade," he added.
Sources close to the brigade say it is divided into smaller groups named after the 12 Shia imams and is mainly composed of Iraqi, Lebanese and Syrian Shias.
Syria's civil war has already attracted hardline Sunni fighters from Afghanistan, Libya and Chechnya, many of whom consider Shia shrines as non-Islamic symbols of paganism.