Amateur videos posted online purport to show Islamic State militants and Syrian regime forces battling for control of a military air base around Deir al-Zor province.
It's one of the last strongholds in the east for Syrian President Bashar al-Assad's government.
Islamic State controls all but a few pockets in the province, whose oilfields have been a major source of revenue for the group.
But its operations have come under strain in recent months...
...ever since a U.S.-led coalition came into the mix and started launching air strikes against them.
The latest battle between Islamic State and Syrian forces comes as a mass grave was revealed in Deir al-Zor province.
The Syrian Observatory for Human Rights says it contains the bodies of more than 230 people killed by Islamic State.
