The Prime Minister has said 'the world should not stand idly by' after hundreds were killed in apparent chemical attacks in Syria last week.
But David Cameron stressed that no decision about British involvement has been made.
He said: "What we've seen in Syria are appalling scenes of death and suffering because of the use of chemical weapons by the Assad regime.
"I don't believe we can let that stand...any action we or others take will have to be legal, proportionate".
He added: "It's about chemical weapons. Their use is wrong and the world should not stand idly by."
ITV News political correspondent Libby Wiener reports:
Earlier, Mr Cameron recalled MPs back to Parliament early so that any potential response to the situation in Syria can be debated and voted on this Thursday.
Deputy Prime Minister Nick Clegg has given his backing to the UK considering a "serious" response to last week's "repugnant crime" in Syria.
Meanwhile, Labour leader Ed Miliband said that the use of chemical weapons on innocent civilians is "abhorrent and cannot be ignored".
Speaking after meeting the Prime Minister in Downing Street, he said his party would consider supporting international action but only on the basis that it was legal.