In a statement issued by the Patriarchates of Antioch and all the East for the Greek Orthodox, Syrian Orthodox, and Greek-Melkite Catholic Damascus, the faith leaders questioned the legality behind the move.
They stated that the "brutal aggression" is a "clear violation of the international laws and the UN Charter", and was an "unjustified assault" on a sovereign country that is a member of the United Nations.
"The allegations of the USA and other countries that the Syrian army is using chemical weapons and that Syria is a country that owns and uses this kind of weapon, is a claim that is unjustified and unsupported by sufficient and clear evidence.
"This brutal aggression destroys the chances for a peaceful political solution and leads to escalation and more complications."
Prime Minister Theresa May has insisted there was no alternative to the air-strikes, following a chemical weapons attack in Douma last week that killed 75 people, including children.
She's being urged to consult Parliament first, if there are any further changes to how the UK deals with Syria.
Labour leader Jeremy Corbyn said: "I think what we need in this country is something more robust like a War Powers Act so that governments do get held to account by Parliament for what they do in our name."
Mr Corbyn warned of an escalation in a "proxy war" between the US and Russia.
The Prime Minister, Donald Trump and Emmanuel Macron have hailed the action a "success".
But the Russian President, Vladimir Putin, says it was an "act of aggression".
Mrs May is continuing to talk to world leaders in the wake of the bombing campaign in Syria as the US warned it was "locked and loaded" for fresh strikes.
She has not ruled out fresh action if Bashar Assad's regime continues to attack its own people.
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