"A coalition warplane fired on a motorbike with two men on it after it left a headquarters" of IS west of Deir Ezzor city in the province of the same name, the head of the Britain-based monitor told AFP.
"One of the men on the bike was an important IS official, an Arab national," Rami Abdel Rahman said, without giving details about his identity or role.
"He was killed, as was the other man," Abdel Rahman said.
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The jihadist group controls much of Deir Ezzor province bordering Iraq, and Abdel Rahman said Friday's strike indicates that the target's movements were being closely monitored.
The United States, driving force behind the growing international coalition against IS, began bombarding the group's positions in Iraq on August 8, and has launched nearly 200 air strikes since then.
On Tuesday, the air strikes spread to Syria, with the active participation of warplanes from five Arab states -- Bahrain, Jordan, Qatar, Saudi Arabia and the United Arab Emirates.
The US and Arab allies launched new strikes on Friday against IS-controlled oil installations in Deir Ezzor and Hasakeh provinces, in addition to other targets.