WASHINGTON — The US State Department said on Saturday reports of mass casualties from an alleged chemical weapons attack in Douma, Syria, were “horrifying” and would demand an international response if confirmed.

“These reports, if confirmed, are horrifying and demand an immediate response by the international community,” State Department spokeswoman Heather Nauert said in a statement.

Pope Francis on Sunday deplored the gas attack as an unjustifiable use of “instruments of extermination”.

“There is no such thing as a good war and a bad war. Nothing, but nothing, can justify the use of such instruments of extermination on defenseless people and populations,” he said at the end of a Mass in St. Peter's Square.

A Syrian rebel group accused government forces on Saturday of dropping a barrel bomb containing poisonous chemicals on civilians in eastern Ghouta, and a medical relief organization said 35 people had been killed in chemical attacks on the area.

The Syrian Observatory for Human Rights said 11 people had died in Douma as a result of suffocation caused by the smoke from conventional weapons being dropped by the government. It said a total of 70 people suffered breathing difficulties.

Medical relief organization Syrian American Medical Society (SAMS) said a chlorine bomb hit Douma hospital, killing six people, and a second attack with “mixed agents” including nerve agents had hit a nearby building.

Basel Termanini, the US-based vice president of SAMS, told Reuters the total death toll in the chemical attacks was 35.