Rebel shelling of one of Syria's two main oil refineries on Wednesday set fire to the plant, already working at barely 10 percent of its capacity, a monitoring group said.

There were no immediate reports of casualties in the bombardment of the plant in the central city of Homs, the Syrian Observatory for Human Rights said.

Syria's oil industry has been massively hit since an uprising against the rule of president Bashar al-Assad erupted in March 2011.

Crude output now stands at barely 10 percent of its pre-uprising levels as a result of European Union sanctions that closed off Syria's main export markets and the rebels' capture of key oilfields that has hit even production for domestic use.

The Homs refinery had a pre-war capacity of 5.7 million tonnes per year but Prime Minister Wael al-Halqi said earlier this month that it was running at 10 percent of that.

The other main plant at Banias on the Mediterranean coast -- heartland of the Assad regime -- is still refining at 80 percent of capacity, Halqi said.

Syria has to look to imports from those countries still willing to supply them, or smuggling, to meet the rest of its needs for refined products.