The Organization for the Prohibition of Chemical Weapons (OPCW), the world’s chemical weapons watchdog, voiced concern on Wednesday over reports of a chlorine gas attack near the Syrian city of Aleppo, AFP reports.

Some 24 people reportedly suffered breathing difficulties in Saraqeb, a town 50 kilometers (30 miles) south of Aleppo, after a barrel bomb attack on Tuesday, according to the Syrian Observatory for Human Rights.

Residents said chlorine gas had been used in the attack, but the Britain-based Observatory could not confirm this.

“These reports are of great concern,” said OPCW head Ahmet Uzumcu in a statement.

The UN-backed group based in The Hague “continues to examine any credible reports” of chemical weapons use, he added.

The incident took place close to where Russia said on Monday one of its military helicopters was shot down, killing the five people on board.

Uzumcu added that under international conventions the use of chemical weapons “by anyone under any circumstances” is seen “as reprehensible and wholly contrary to the legal norms established by the international community”.

Chemical weapons have been repeatedly used in the Syrian civil war. Syria agreed to destroy its chemical weapons in 2013 under a deal brokered by Moscow and Washington, but the OPCW has since found chlorine has been “systematically and repeatedly” used as a weapon.

Government and opposition forces have denied using chlorine and have accused each other of doing so.

The government argues that claims that it used chemical weapons are only meant to ” serve political agendas”.

In January, the OPCW announced all Syria’s declared chemical arms stockpile had been completely destroyed, but concerns remain that undeclared amounts of sarin gas and other chemical

weapons have still been used in the conflict.

Last month the chemical weapons watchdog urged Syria to explain why it has four undeclared warfare agents, after an American official accused Damascus of continuing to hoard a toxic stockpile.

A joint investigation set up by the UN probing nine chemical weapons attacks in Syria in 2014 and 2015 is due to report its findings this month, noted AFP.

In addition to chemical weapons attacks attributed to the regime, there have also been attacks attributed to jihadist groups.

American intelligence officials have said they “knew” the Islamic State (ISIS) was carrying chemical attacks in both Syria and Iraq, adding the jihadists used such weaponry “on at least four separate occasions on both sides of the border.”