While the oil and gas fields are in serious decline, control of them has bolstered the fortunes of the Islamic State in Iraq and the Levant, or ISIL, and the Nusra Front, both of which are offshoots of Al-Qaida. ISIL is even selling fuel to the Assad government, lending weight to allegations by opposition leaders that it is secretly working with Damascus to weaken the other rebel groups and discourage international support for their cause.

Although there is no clear evidence of direct tactical coordination between ISIL and Assad, U.S. officials say that his government has facilitated the group’s rise not only by purchasing its oil but by exempting some of its headquarters from the airstrikes that have tormented other rebel groups.

The Nusra Front and other groups are providing fuel to the government, too, in exchange for electricity and relief from airstrikes, according to opposition activists in Syria’s oil regions.

The scramble for oil is described by analysts as a war within the civil war, one that is turning what was once an essential source of income for Syria into a driving force in the conflict.

“Syria is an oil country and has resources, but in the past they were all stolen by the regime,” said Abu Nizar, an anti-government activist. “Now they are being stolen by those who are profiting from the revolution.”

He described the situation in his oil-rich province as “overwhelming chaos.”

The Western-backed rebel groups do not appear to be involved in the oil trade.

Syria was once an important supplier of oil to Europe, and attracted international oil companies like Royal Dutch Shell and Suncor to develop its fields. Declining even before the anti-Assad uprising began, the oil industry has taken a beating since, with production down to no more than 80,000 barrels a day at the end of 2013 from about 400,000 barrels a day in 2011.