Donald Trump has raised the prospect of military action against the Syrian regime after he said that Bashar al-Assad has "crossed a lot of lines" and the US has a "responsibility" to act.

The US president said Tuesday's chemical weapons attack, which intelligence officials say was undoubtedly launched by regime war planes, was an "affront to humanity".

At least 73 people were killed, 23 of whom were children, in the sarin gas attack in the rebel-held Idlib province.