The U.N. envoy for Syria urged its close ally Russia and world leaders on Friday to try to overcome President Bashar Assad's opposition to the formation of a committee to draft a new constitution that is key to ending the country's civil war.

Staffan de Mistura told the Security Council the U.N. faces "a serious challenge" as a result of the government's objection. But he expressed hope that high-level meetings planned in the coming weeks will spur a solution and enable him to launch a 150-member committee to draft a constitution before he steps down in late November.

De Mistura welcomed an invitation to brief the leaders of Russia, France, Germany and Turkey in Istanbul on Saturday and said he will ask them to "seize the opportunity" of relative calm in rebel-held Idlib and use their influence to spur a speedy solution on the composition of the constitutional committee.

"The influence that can be exercised by all world leaders including very much the four world leaders who I am meeting tomorrow in Istanbul can be crucial in ensuring that this happens," he said.

At issue is the 50-member delegation de Mistura was authorized to put together at a Russian-hosted Syrian peace conference in the Black Sea resort of Sochi on Jan. 30 representing Syrian experts, civil society, independents, tribal leaders and women. Agreement has been reached on 50-member delegations from the government and from the opposition for the drafting committee.

De Mistura told the Security Council by video link from Beirut that at a meeting in Damascus on Wednesday, Syrian Foreign Minister Walid al-Moallem "did not accept a role for the U.N. in general in identify or selecting" members of the list he made.

He said al-Moallem indicated that Syria and Russia had recently agreed that the three guarantor states in the so-called "Astana process" aimed at ending the violence in Syria — Russia and Iran which support the government and Turkey which backs the opposition — would instead prepare a list and present it to the U.N.

De Mistura said he looks forward to hosting the Astana guarantors "very soon" which he said will be "a vitally important consultation."

He said he will also be meeting with the so-called Small Group in London on Monday comprising the United Kingdom, United States, Egypt, France, Germany, Jordan and Saudi Arabia.

U.S. deputy ambassador Jonathan Cohen said the Small Group's members "are united in our position that the U.N. should move swiftly to convene the constitutional committee," stressing that "further obstruction on the committee's formation is unacceptable."

De Mistura said "the U.N. is not opposed to constructive and moderate suggestions" on the list he drew up, but it must maintain "credibility, balance, international legitimacy."

France's U.N. Ambassador Francois Delattre said "Syria is currently at a crossroads" and it can tip into escalation in Idlib or take "the path of political momentum" starting with agreement on a constitution drafting committee.

"Between war and peace in Syria, the key is largely in the country of Tolstoy," he said, referring to the famed Russian novelist Leo Tolstoy who wrote "War and Peace."

Britain's U.N. Ambassador Karen Pierce said there is now "enormous doubt" over the Sochi agreement.

"Either Russia has given the U.N. and this council assurances it has proved too weak to deliver on, or it was all a cynical smokescreen designed to divert attention and energy while Russia, Syria and Iran prosecuted the military campaign," she said.

The more than seven-year Syrian conflict has killed over 300,000 people and the latest military campaign has put the government in control of most Syrian territory. An agreement between Russia and Turkey in September averted an expected Syrian military offensive in Idlib, but there are fears it may not hold.

Russia's U.N. Ambassador Vassily Nebenzia accused the U.S., Britain and France, who called Friday's meeting, of convening "another show set up with tragic wails and moralizing against Russia."

He stressed that "there are no grounds for establishing artificial deadlines for the establishment of a constitutional committee," saying patience is required.

But Nebenzia also said Russia is confident that before de Mistura steps down he "will leave no stone unturned to make progress on this area, within his mandate as mediator, with full respect to the sovereignty of Syria, and we will help him in doing that."

Syria's U.N. Ambassador Bashar Ja'afari stressed that "the role of the United Nations is one of a facilitator — to facilitate inter-Syrian dialogue, rather than having guardianship over this dialogue."

The government believes that creating a constitutional committee reflecting Syrian society "is a way of exiting the crisis," he said.