* Brigade formed in the summer in response to Sunni threats to destroy shrine
Shi'ite fighters from Iraq and Lebanon have joined fellow Shi'ite Syrian gunmen to defend a shrine south of Damascus which they fear is threatened by Sunni rebels battling President Bashar al-Assad.
The presence of Shi'ite combatants from neighbouring states - confirmed by sources in Iraq and Syria and highlighted in videos glorifying their mission - underlines how Syria's conflict is inflaming sectarian feelings in the region.
Abu al-Fadl al-Abbas brigade, named after a seventh century martyr son of Imam Ali who is considered the father of Shi'ite Islam, was formed several months ago and fights mainly around the shrine of Sayyida Zeinab on the southern outskirts of the Syrian capital, a source close to the brigade said.
Abbas's sister Zeinab is buried in the gold-domed mausoleum, intricately decorated with blue ceramic tiles and surrounded by a white marble courtyard which used to fill with pilgrims before the uprising against Assad erupted and grew into a civil war.
The source said the brigade was set up in response to the perceived danger to the shrine and mosque from Sunni fighters who desecrated other places of worship for Shi'ites, who are a minority in Syria.
"They are there for one purpose and that is to defend the shrine," he said, adding they were operating independently of Assad's forces around the capital.
He said Iraqi fighters at Sayyida Zeinab were motivated partly by the desire to prevent a repeat of the wholesale sectarian violence that followed the 2006 attack on the Iraq's Shi'ite Imam al-Askari mosque, blamed on al Qaeda, which cost thousands of lives, both Sunni and Shi'ite.
Syria's conflict has already attracted hardline Sunni fighters some from Afghanistan, Libya and Chechnya, many of whom consider Shi'ites infidels and their shrines as non-Islamic symbols of paganism which should be torn down.
A video posted online two months ago showed Sunni Muslim rebels burning a husseiniya - a Shi'ite religious site - in Syria's northern Idlib province, one of several recent attacks against properties associated with religious minorities.
An Iraqi Shi'ite official said Iraqi Shi'ites - some of whom had lived in southern Damascus since fleeing Iraq's own violence - started to mobilise last summer in response to rebels in the area he described as "hardliners and Salafis", referring to the ultraconservative school of Sunni Islam.
Rebels "wanted to destroy the Sayyida Zeinab shrine and hundreds of Iraqi Shi'ites who were already living in Syria stood up to them and fought back," he told Reuters from Iraq.
"Now they are more organised, under the Abu al-Fadl al-Abbas Brigade," he said. Sources close to the brigade say it is divided into smaller groups named after the 12 Shi'ite imams and is mainly composed of Iraqi, Lebanese and Syrian Shi'ites.
The brigade is still made up mainly of Iraqis, the official said, though he said they came to Damascus individually and not under the auspices of the state or any organisation.
Rebels accuse Lebanon's Hezbollah, an ally of Assad, of fighting alongside his forces. The group denies the accusations and says only that its loyalists are fighting in border villages to defend Shi'ites there.
"CUT OFF THE HANDS"
The brigade has put out two videos that play heavily on the historical feud between Sunnis and Shi'ites. The first, called "O Zeinab", shows the shrine damaged with a chandelier on the floor. "We will cut off the hands of the perpetrators," says a chant on the soundtrack.
The videos mix recent footage of the current conflict around Sayyida Zeinab with scenes from a drama portraying Abbas's death in 680 AD at the hands of the Damascus-based Umayyad Caliph Yezid's army at the battle of Kerbala, in modern-day Iraq.
Those images reinforce the sense of a conflict that transcends state borders. While Shi'ites form barely 2 percent of the population in Syria, they are a majority in Iraq and Iran and a strong force in neighbouring Lebanon - countries where sympathy for Syria's Shi'ite and Alawite minorities runs deep.
"The Umayyad descendents are back with their injustice, O Zeinab," chants an Iraqi-accented voice in the first video, released in December.
Fighters in camouflage uniform, their faces blanked out, fire rocket-propelled grenades and shoot automatic rifles during apparent street battles. Some take up sniper positions, and all of them seem well-trained. Destruction and piles of rubble can be seen in front of closed shops.
It ends with a black-clad Zeinab addressing Yezid. "You will not succeed in erasing our memory," she says.
In the second video released this month the chanter says: "We will not allow (Zeinab) to be captive twice," a reference to her capture after the battle of Kerbala.
One of the fighters filmed in the first video is shown in the second, this time with his face uncovered - because he has been killed in battle, becoming "a martyr defending Zeinab".
At least six fighters are seen shooting from a roof while others are seen praying inside the shrine. The tone of the video is more defiant, the "enemy" now mentioned by name.
"If we receive the order ... we will turn things upside down and burn Damascus," it says. "You Free Army (the rebel army name) get ready. We are coming, we are coming."
The anger on display is mirrored on social media by Sunni fighters and reflects the deepening rifts across Syria between majority Sunnis, Alawites and Shi'ites who are being driven further and further apart by the violence.
In a video posted on YouTube last July a Sunni commander warns residents of the northern village of Binnish not to deal with Shi'ites in neighbouring villages, warning that even trading bread or other goods with them is punishable by death.
"The people of Foa (village) are Shi'ites and they are our enemy across the globe. Understand this. Whoever deal with them even if it is a grain, a single grain of wheat, his punishment will be the same," the rebel commander says, to the chants of Allahu Akbar (God is greatest).
"I swear, I swear, I swear - if it is proven that a man from this village is dealing with them I will kill him at the door of the mosque," he said.