“While most attention has been given to Sunni terrorist groups such as the Islamic State of Iraq and al-Sham (ISIS), which is battling Iraqi government forces in Anbar province, there are also worries about those Iraqi Shiite fighters who have joined the fight on behalf of Assad. US Secretary of State William Burns testified to the US Senate Foreign Relations on March 6, ‘Assad has recruited thousands of foreign fighters, mostly Shia, to defend the regime, with active Iranian support and facilitation. The hard reality is that the grinding Syrian civil war is now an incubator of extremism — on both sides of the sectarian divide.'”

Source: Iraqi Shiite militants return from Syria, aggravating security crisis – Al-Monitor: the Pulse of the Middle East