Rebels are reported to have seized the largest oilfield in Syria, cutting off President Bashar al-Assad's access to almost all domestic crude reserves.
There has been no immediate comment from the Assad government and the claim by the Syrian Observatory for Human Rights is yet to be verified independently.
But if confirmed, the loss of the al-Omar field in Deir al-Zor province could leave government forces almost completely reliant on imported oil.
"Now, nearly all of Syria's usable oil reserves are in the hands of the Nusra Front and other Islamist units ... The regime's neck is now in Nusra's hands," said Observatory head Rami Abdelrahman in London.
Government forces have gained momentum against the rebels in recent months, partially due to support from Hizbollah fighters from Lebanon and its regional ally Iran.
But opposition fighters still hold large areas of territory in northern and eastern Syria. The Nusra Front has been linked to al Qaeda.