UN secretary-general Ban Ki-moon has urged world leaders to stop fuelling the bloodshed in Syria with weapons and get both sides to the negotiating table to end the "biggest challenge to peace and security in the world".
In his state of the world address to open the annual gathering of presidents, prime ministers and monarchs at the UN General Assembly, the UN chief said the international response to last month's "heinous use of chemical weapons" in Syria "has created diplomatic momentum - the first signs of unity in far too long."
Mr Ban called on the Security Council to adopt an "enforceable" resolution on a US-Russian agreement to put Syria's chemical weapons under international control for destruction and bring to justice the perpetrators of the August 21 chemical weapons attack outside Damascus "either through referral to the International Criminal Court or by other means consistent with international law".
UN diplomats say differences between the US and Russia on how a resolution should be enforced have held up action in the Security Council. Russia is opposed to any mention of Chapter 7 of the UN Charter, which includes military and non-military actions to promote peace and security. Russia and China have vetoed three Western-backed resolutions that would have pressured Syrian president Bashar Assad to end the two-and-a-half year war that has killed more than 100,000 people.
US president Barack Obama noted the Iranian and Russian support for the Syrian regime.
"It's time for Russia and Iran to realise that insisting on Assad's rule will lead directly to the outcome they fear: an increasingly violent space for extremists to operate," Mr Obama said. "In turn, those of us who continue to support the moderate opposition must persuade them that the Syrian people cannot afford a collapse of state institutions, and that a political settlement cannot be reached without addressing the legitimate fears of Alawites and other minorities."
Turkish president Abdullah Gul welcomed the US-Russian agreement to destroy Syria's chemical weapons but said that should not allow those who perpetrated a "crime against humanity" by using the weapons against civilians to escape justice.
Mr Gul, whose nation borders Syria to the north and hosts refugees from the conflict, also lamented that "geopolitical considerations" had stymied Security Council action to stop the fighting.
"It is a disgrace that the United Nations Security Council has failed to uphold its primary responsibility in this case," he said, predicting that if the international community fails to act, the death toll would double by next year.
Jordanian King Abdullah II said Syrian refugees have overwhelmed his nation and now amount to one-tenth the size of his country's population. He said that could grow to 20% next year and called on the international community to "fast-track a political transition in Syria".