European countries including Britain are to review the arms embargo in Syria as part of continuing efforts to support the opposition to President Assad, David Cameron has told the Commons.
The Prime Minister told MPs last week's gathering of European leaders had agreed there was a "moral imperative" for action in Syria, which has seen bloody fighting for more than a year.
Mr Cameron said Britain was leading the way among international efforts as the second-largest donor to humanitarian aid.
And reporting on the European Council in a statement to MPs, Mr Cameron said: "A humanitarian crisis is unfolding in Syria on our watch, with over 40,000 dead and millions in need of urgent assistance as a hard winter approaches.
"We continue to encourage political transition from the top and to support the opposition which is attempting to force a transition from below.
"This should include and will include looking at the arms embargo. We must now explore all options to support the opposition to enable greater support for the protection of civilians."
Mr Cameron said there was also a strategic imperative to act because of a "growing cohort of al Qaida-linked extremists" in the country meaning there was a growing risk of regional instability.
He said: "We cannot go on as we are. Assad's regime is illegitimate, the council committed to work for a future for Syria that is democratic and inclusive with full support for human rights and minorities."
Answering Labour leader Ed Miliband, Mr Cameron added: "It is right to look at amending the arms embargo; of course, we will be keeping the arms embargo on the regime. There are arguments on both sides of the debate but it is a debate that should be had, that European foreign ministers will be having."
Responding to the statement, Mr Miliband said: "The international community must work together to end the atrocities immediately and speak with one voice in favour of a transition to a new government."