Dozens were killed when a Syrian gas field was overrun by militants from the group formerly known as ISIS earlier this week, according to an NGO.

The death toll was estimated to be at least 90 by the Syrian Observatory for Human Rights on Friday.

The figure includes Syrian national soldiers, workers and security guards although the SOHR said 270 more people were unaccounted for.

The Sha’ar gas field is in eastern Homs. Most of Syria’s gas fields cluster in the centre of the country.

Last week an official from the Syrian opposition government claimed ISIS – which recently declared its own “caliphate”, claiming to be leader of all Muslims – now controlled more than 60% of Syria’s oil, reported news site Al-Awsat.