A non-governmental organization says at least 30 people were killed Sunday during a U.S.-led airstrikes on an Islamic State group-controlled oil refinery in northern Syria.

According to reports from the UK-based Syrian Observatory for Human Rights, several militants were killed in the attack, however the rest of the victims were civilians and workers. Syrian authorities have not yet made an official statement.

The Islamic State group controls some of the oil-producing regions of Syria, and the U.S.-led coalition regularly targets the oil infrastructure there, saying the extremist organization smuggles oil for part of its funding.

However critics say the attacks are also destroying the only civilian economic infrastructure in those regions.

Washington and its Gulf allies began their bombing campaign against the Islamic State last summer and since late September their forces have been conducting airstrikes against the extremist organization's positions in Syria and Iraq without any authorization from the Syrian government or the U.N. Security Council.

The coalition has conducted more than 2,300 airstrikes in Iraq and Syria so far, according to data from the U.S. Defense Department.

Syria has been gripped by deadly violence since 2011. According to international organizations on the ground, more than 191,000 people have been killed in the over three years, mostly in fighting between the Syrian army and western-backed armed opposition groups looking to oust the government of President Bashar al-Assad.