Major fighting erupted in the oil-rich city of Bukamal, in Syria’s Deir Ezzor Province, as al-Qaeda in Iraq (AQI) attempted to wrest control from its rival al-Qaeda faction, Jabhat al-Nusra.

The city is doubly valuable for rebel factions, as it is both a source of oil revenue for its occupier and has a border with Iraq’s Anbar Province, a particularly big incentive for AQI, which presently controls much of Anbar as well.

At least 51 people have been reported killed so far as AQI has gained considerable ground. Nusra fighters have reportedly requested reinforcements in an attempt to keep the city.

AQI and Jabhat al-Nusra have been openly warring for months, since al-Qaeda’s parent organization disavowed AQI as too brutal for their taste, and endorsed Nusra as their official Syria affiliate. Despite losing their imprimatur, AQI has continued to control much more territory and seems to be the stronger of the two factions.