The committee is part of the Sochi agreement and will include 50 members from the regime, 50 opposition members, the rest will be independent.
The foreign ministers of Russia, Iran and Turkey said on Tuesday that a new Syrian Constitutional Committee should convene early next year, kicking off a viable political peace process.
In a joint statement read out by Russian Foreign Minister Sergei Lavrov after the trio met UN Special Envoy Staffan de Mistura, they said that the work of the new body "should be governed by a sense of compromise and constructive engagement."
The committee is part of the Sochi agreement and will include 50 members chosen by the regime, 50 opposition members, and 50 by the United Nations.
The final list will be presented to outgoing de Mistura.
In a separate statement, de Mistura said that after the tripartite talks with Russia, Iran and Turkey "there is an extra mile to go in a marathon effort" to ensure a credible, balanced constitutional committee for Syria.
The centrepiece of UN peace efforts in Syria, the committee would be tasked with negotiating a new post-war constitution that would pave the way to elections aimed at turning the page on seven years of devastating war.
But it has run into objections from the Syrian regime.
The opposition has pushed for an entirely new constitution, but the regime has said it will only discuss altering the current one.
In October, Damascus rejected a list presented by de Mistura of 50 civil society representatives and technical experts.
The spokesman for the Syrian negotiation commission, Yahya al Aridi, elaborates on the challenges in putting this committee together.
De Mistura to deliver one last push
"You cannot win a war full stop," de Mistura stressed.
But peace could be accomplished "through a credible inclusive constitution", he added.
De Mistura, an Italian-Swedish diplomat who has been the UN's peace envoy since July 2014, was due to step down at the end of November, but he agreed to stay on for an extra month to lead a final push.
Last month he said the UN is still hoping to send invitations to committee members by mid-December and convene a first meeting before December 31.
Russia, Turkey, Germany and France have called for the committee to be formed by the end of the year.