BEIRUT (REUTERS) - A US-led coalition air strike in Syria hit an oil refinery run by the Islamic State in Iraq and Syria (ISIS) militant group near the border with Turkey on Sunday, killing 30 people, the Syrian Observatory for Human Rights reported.

Rami Abdulrahman, who runs the Observatory, said the dead were refinery workers and ISIS militants. The targeted refinery was just north-east of the town of Tel Abyad near the Turkish border, he said.

The ISIS group has seized wide areas of Syria and Iraq, declaring them part of a cross-border "caliphate". The territories it controls in northern and eastern Syria include oil-producing regions that have financed the group's activities.

In November, the United Nations estimated that ISIS' revenue from oil ranged between US$846,000 and US$1.6 million (S$1.2 million and S$2.2 million) a day.

However, the Pentagon has assessed that oil was no longer the main source of revenue for ISIS. Western diplomats have said this was due to air strikes on oil installations and a plunge in global oil prices that has affected black market prices as well.