Foreign Secretary William Hague has warned it would be a catastrophe if a United Nations (UN) conference failed to reach a peace plan for Syria.
Mr Hague called for a UN Security Council resolution to back up ceasefire proposals.
The UK and other western powers have said Syrian president Bashar Assad should be replaced with a democracy but Russia has demonstrated a determination to preserve its last remaining ally in the Middle East.
Syria envoy Kofi Annan has warned that failure to end the country's violence would result in an international crisis of "grave severity".
Mr Hague told the Geneva summit: "We face a heavy responsibility today. The world is looking to us for leadership and action to end the bloodshed and horror in Syria. We should heed the Secretary General's words on this.
"We have a choice: we can unite around a robust and effective plan to achieve a ceasefire and a political transition in Syria, and we can agree to give this plan the force and backing of a UN Security Council resolution. Armed with that, we can launch a concerted attempt to halt the violence once and for all.
"Or we can fail to overcome these differences, miss the opportunity to achieve a diplomatic breakthrough and watch the situation deteriorate further. Of course, we benefit from having a six-point plan in front of us.
"The cost of any such failure would be counted by the people of Syria in lives lost and injuries sustained, livelihoods wrecked and suffering endured. I think it would embolden all those who think they can fight their way to victory, and give space for extremist groups to establish a foothold. It could turn a humanitarian crisis into a catastrophe."
Mr Hague stressed the importance of a resolution, claiming it was not possible to get Syria to change course without one.
He added: "President Assad and his closest associates cannot credibly lead the process of transition in Syria. This is a statement of fact, reflecting reality now that so much blood has been spilt. Their failed leadership is now the prime cause of the instability and crisis in Syria. Their involvement would condemn transition to failure before it had begun, since it inevitably would be seen as an attempt to cling to power."