A US-led coalition air strike in Syria hit an oil refinery run by the Islamic State and the Levant (ISIL) group near the border with Turkey, killing 30 people, the Syrian Observatory for Human Rights has said.

Rami Abdulrahman, who runs the UK-based monitoring network, said on Sunday the dead were refinery workers and ISIL fighters.

The targeted refinery was just northeast of the town of Tel Abyad near the Turkish border, he said.

Asked about the report, a spokesperson for the US Combined Joint Task Force overseeing the fight against ISIL, said: “When an allegation of civilian casualties caused by US or coalition forces is determined to be credible, a thorough investigation would be launched to determine the accuracy of the claim and any circumstances surrounding it.”

The spokesperson said US and coalition forces had implemented “significant mitigation measures within the targeting process and during the conduct of operations to reduce the potential of civilian casualties and collateral damage”.

ISIL 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 UN estimated ISIL’s revenue from oil ranged between $846,000 to $1.6m a day.

But the Pentagon has assessed that oil was no longer the main source of revenue for ISIL. Al Jazeera