There is a report in the Lebanese Daily Star* that ISIS had seized parts of the Yarmouk refugee camp in Damascus. I've also seen a report that the rebels have beaten them back. The Daily Star says ISIS are opposed to Assad, but I don't believe his forces will have done anything to impede this attack on his real enemies.
"Four, recent military gains made by the rebels with support from Jabhat al-Nusra are a fatal blow to both Bashar al-Assad and ISIS. The Islamic State has long accused the rebels and Jabhat al-Nusra of betraying Islam and claimed that they could never defeat Assad. However, the fact that Jabhat al-Nusra reached an understanding with Syrian rebel factions and their subsequent achievements in Idlib are a big loss for the Islamic State. “ISIS fears Nusra and the Salafist groups more than Assad and the international coalition—those groups have taken away its religious legitimacy,” says Sharia graduate Sheikh Qassem. “They have proved that there is another Islamist project that is not at odds with the goals of the Syrian revolution, or that it can achieve harmony with the revolution during the current period at least. In fact, they have proved that it is wrong to monopolize religion and use it for political reasons that have nothing whatsoever to do with religion itself.” " *[http://www.dailystar.com.lb/News/Middle-East/2015/Apr-01/292956-isis-seizes-most-of-yarmouk-refugee-camp-in-damascus-plo.ashx#.VRvrgJvrXsY.twitter]