Syrian rebels seized one of the country's major oilfields in the eastern province of Deir Ezzor on Sunday, a first for the opposition, the Syrian Observatory for Human Rights said.
"Rebels in the Jaafar Tayyar Brigade took control of Al-Ward oilfield, east of the town of Mayadin, after a siege that lasted several days," the Britain-based watchdog said.
"This is the first time the rebels have taken control of an oilfield," Observatory director Rami Abdel Rahman told AFP by phone.
The fighting on Sunday began at dawn and lasted several hours, said Abdel Rahman, adding that 40 soldiers guarding the infrastructure had either been killed, wounded or taken prisoner.
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The rebels, who face the superior firepower of regime forces, also seized a tank, armoured vehicles and ammunition.
Several attacks have targeted the country's oil infrastructure since the uprising against the regime of President Bashar al-Assad broke out in March 2011.
Deir Ezzor province, which borders Iraq, contains the largest energy reserves in the country, and Al-Ward field is one of the most important in the province.
Syria produced some 4,000 of barrels of oil a day before the United States and the European Union banned in August and September 2011 the import of Syrian petroleum and petroleum products to put pressure on the Assad regime.
Oil exports fell from 13,500 tonnes in the fourth quarter of 2011 to 7,500 tonnes in the first quarter of 2012.
Europe had previously bought 95 percent of Syrian oil, generating a third of the country's revenue.