Pope Francis on Sunday deplored the gas attack reported in Syria 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 defenceless people and populations," he said at the end of a Mass in St. Peter's Square.
He urged that "military and political leaders choose another path, that of negotiations, which is the only one that can bring about peace and not death and destruction".
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A chemical attack on a rebel-held town in eastern Ghouta has killed dozens of people, medical services reported, and Washington said the reports - if confirmed - would demand an immediate international response.
A joint statement by the medical relief organisation Syrian American Medical Society (SAMS) and the civil defence service, which operates in rebel-held areas, said 49 people had died in the attack late on Saturday. Others put the toll at 150 or more.
The Russian-backed Syrian state denied government forces had launched any chemical attack as the reports began circulating and said the rebels were collapsing and fabricating news.
Reuters could not independently verify the reports.
The lifeless bodies of around a dozen children, women and men, some of them with foam at the mouth, were shown in one video circulated by activists. "Douma city, April 7 ... there is a strong smell here," a voice can be heard saying.
The U.S. State Department said reports of mass casualties from the attack were "horrifying" and would, if confirmed, "demand an immediate response by the international community".