"Colonel Moamer Qadhafi has made this happen. He has lied to the international community, he has promised a ceasefire and he has broken that ceasefire. He continues to brutalise his own people," Cameron told British television.
"And so the time for action has come. It needs to be urgent. We have to enforce the will of the United Nations and we cannot allow the slaughter of civilians to continue."
Cameron was speaking from Paris where US, European and Arab leaders had gathered to discuss Libya following the United Nations resolution demanding a ceasefire and approving a no-fly zone.
He later returned to London, where he was chairing a meeting of the government's crisis response committee, known as COBRA.
In his comments in France, the British leader said it was "a successful coming together of countries including Arab countries who want to enforce the will of the United Nations".
"What is absolutely clear today is that Colonel Qadhafi has broken his word, has broken the ceasefire and continues to slaughter his own civilians," he said.
"This has to stop. We have to make it stop; we have to make him face the consequences.
"I think it is vitally important that action takes place, that action takes place urgently."
He added that "of course there are dangers, there are difficulties, and there will always be unforeseen consequences from taking action."
"But it is better to take this action than to risk the consequences of inaction, which is a further slaughter of civilians and this dictator completely flouting the United Nations and its will," he said.