The Syrian Observatory for Human Rights in London and the Local Coordination Committee said the airstrike occurred in a field outside Idlib in northern Syria, The New York Times reported.
Anti-government activist Ahmad Kadour told the Times a Syrian warplane dropped two bombs on the Abu Hilal olive oil press.
"I'm sure we'll find out more people died, but it's too soon to find out the real number," he said.
The government of Syrian President Bashar Assad, which has been fighting the popular uprising since early 2011, had no immediate reaction to the rebel claims, the U.S. newspaper said.
Rebel forces, meanwhile, maneuvered to solidify advances they had made Monday, including the takeover of a dam on the Euphrates River.