Militants from the Islamic State group seized control of Syria's largest oil field al-Omar from rival rebel fighters on Thursday, strengthening its advance across the eastern Deir al-Zor province, a monitoring group said.

"They took leadership about two hours ago," said Rami Abdelrahman, head of the Syrian Observatory for Human Rights. Nusra Front, Syria's al Qaeda wing, had previously been in control of the field.

Syria is not a significant oil producer and has not exported any oil since late 2011, when international sanctions took effect to raise pressure on President Bashar al-Assad. Before sanctions, Syria exported 370,000 barrels per day, mainly to Europe.