At least eight people were killed in at least 50 air strikes on Syria's eastern Deir Ezzor province Friday, during which dozens of oil tankers were destroyed, a monitor said.
The Syrian Observatory for Human Rights said the raids were carried out either by Syrian or Russian war planes.
The Britain-based Observatory said it had documented at least 50 strikes in different parts of the oil-rich province, large parts of which are controlled by the Islamic State group.
It said dozens of oil tankers and other vehicles used for transporting crude had been destroyed.
"This is the first time Deir Ezzor has experienced strikes of this intensity," the monitor said.
The strikes come after the US-led coalition fighting IS said earlier this week it had destroyed 116 fuel trucks used by the jihadists in Albu Kamal, an IS-held town in Deir Ezzor.
A coalition spokesman said that was part of a strategy to "start degrading (IS's) financial ability".
The coalition began strikes in Syria last year, and Russia launched its own aerial campaign there on September 30.
Also in Deir Ezzor, the Observatory said fierce fighting was ongoing between regime forces and IS militants who tried to storm the military airport in the province, which is held by the regime.
IS militants had failed to enter the airport, but fighting was ongoing outside it and at least eight regime forces and 22 IS fighters had been killed.
Most of Deir Ezzor is held by IS, but the regime has clung onto the military airport and the provincial capital.