Pope Francis has condemned the use of chemical weapons in Syria and prayed for all who are suffering the consequences.

"There is no good and bad war, and nothing, nothing can justify the use of such instruments of extermination against defenseless people and populations," he said today in Rome at Holy Mass on the second Sunday of Easter.

He prayed for all the dead, for the wounded and for the families who suffer.

He said: "We pray that political and military leaders choose the other way: that of negotiation, the only one that can lead to a peace that is not that of death and destruction."

He spoke reports emerged of a chemical attack on a rebel-held town in eastern Ghouta that killed dozens of people. Washington said the reports – if confirmed – would demand an immediate international response (Reuters reports).

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".

Britain's Foreign Office also called the reports, if confirmed, "very concerning" and said "an urgent investigation is needed and the international community must respond. We call on the Assad regime and its backers, Russia and Iran, to stop the violence against innocent civilians."