The advocacy group In Defense of Christians also condemned the attack.
"IDC condemns in the strongest terms the brutal violence and tactics of torture being used by Syrian President Bashar al-Assad's regime with the support of Russian and Iranian forces against civilians in Syria," IDC executive director Philippe Nassif stated.
Religious and political leaders have called for prayer, for the perpetrators to be held accountable, and for a peaceful end to the conflict which has lasted for over six years.
"In this season of Lent when Christians draw near to the suffering of Christ, let us match the horrific indifference shown for innocent life with a fervent prayer for love to break through the evil," Cardinal DiNardo said. "Let us also match our prayer with a faithful witness to suffering so that no life at risk is forgotten."
Pope Francis condemned the attack and offered his prayers "for the victims and their families."
"I appeal to the conscience of those who have political responsibility, locally and internationally, so that this tragedy may come to an end and relief be brought to that beloved population who for too long have been devastated by war," he said.
The use of poison gas on civilians is a war crime, but under international criminal law the perpetrator is not known right now with complete certainty, Professor Robert Destro, an international law expert at the Catholic University of America's Columbus School of Law, explained to CNA.
"We need to know who did it," he insisted. "There are certain things that you are just not allowed to do."
There are several actions countries could take to find the perpetrators and hold them accountable, he said, one approach being to use international criminal law, filing an indictment against the guilty party in an international criminal tribunal.
However, for this to take place, world leaders like the U.S., China, and Russia would have to come together, as "the U.S. cannot hold them responsible by itself."
The international community must start meeting to determine not the best interests of particular countries, he said, but rather "what is in the best interest of people who live in the neighborhood" in Syria.
(Story continues below)
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