US president Donald Trump says the US is "locked and loaded" if the Syrian regime launches another chemical weapons attack on its own people.

The president issued the threat after the US, UK and France launched co-ordinated air strikes against Syria in retaliation for a chemical attack on rebel-held territory in Douma.

Syria's biggest ally, Russia, claims the strikes were illegal and has rejected claims that President Bashar al-Assad's regime was responsible for the attack.

But Downing Street has published a document saying the strikes met international laws for military action on humanitarian grounds and they were launched to avert a "humanitarian catastrophe".

Unidentified warplanes bombed four Assad regime sites on Saturday night, including an Iranian military base near Aleppo, according to reports.

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