David Cameron has condemned the "completely disgraceful and unacceptable" violent crackdown on protests in Syria.
The Prime Minister said international pressure on President Bashar Assad's regime needed to be stepped up.
Security forces have killed hundreds of people since the protests in Syria began, many of them in the besieged southern city Deraa.
Speaking on BBC1's Andrew Marr Show, the Prime Minister said: "It is a completely disgraceful and unacceptable situation, to see the regime killing so many of its own people."
Pressed on why the UK and international allies had intervened in Libya but were not taking action against Syria, Mr Cameron said: "There are some differences.
"In Libya we were asked by the Arab League to go into that country, we were asked by the Libyan people, we were backed by a United Nations resolution.
"Clearly in Syria we need to do more to step up the pressure on that regime to show, internationally, that is not acceptable.
"We have started that process in the European Union but I think we have further to go and more to do."