Staffan de Mistura, the U.N. special envoy for Syria, told the Security Council that the U.N. also aims to hold the committee's first meeting before Dec. 31.
But de Mistura said the Syrian government is objecting to 50 members of the committee representing civil society, experts, independents, tribal leaders and women that he was authorized to put together at a Russian-hosted Syrian peace conference in Sochi on Jan. 30.
Under the Sochi agreement, the committee is to comprise 150 members. There is already agreement on the 50-member delegation from the government and the 50-member delegation from the opposition.
But De Mistura warned that if there is no agreement on the remaining members, the U.N. may have to conclude that it's not possible to form a "credible and inclusive" constitutional committee at this stage.
He said the U.N. welcomes "constructive and moderate suggestions" to change the list of the disputed 50 members. But de Mistura said they must "maintain the same spirit of credibility, balance and international legitimacy," and he stressed that the list can't be filled with political leaders who are already represented.
The U.N. envoy said that at his last briefing to the Security Council in December "it will be my duty to explain where we are on the constitutional committee, and leave a clean and clear ground to my successor regarding it."
De Mistura was supposed to step down at the end of December but U.N. spokesman Stephane Dujarric said Monday he will be staying on "for a bit longer" to make sure there is no gap "at an extremely critical time in the Syria talks." He will be succeeded by veteran Norwegian ambassador Geir Pederson.
"The weeks to come will be of crucial importance" in attempts to form a constitution-drafting committee, de Mistura said.
U.S. deputy ambassador Jonathan Cohen urged the constitutional committee to be convened in December — "if not sooner."
He said de Mistura has been "more than patient and flexible" in accepting input on the list of committee members.
"Any further deliberation must be seen for what it is: an artificial delay, a transparent attempt to distract and delay progress toward a political solution in Syria," Cohen said.
He said the Syrian government and its allies Russia and Iran can't be allowed to veto de Mistura's decision on the committee's membership.
"The burden is on Russia to show that it wants its own diplomatic initiative to succeed," Cohen said. "Russia must bring the Syrian regime to the table. If it truly wants a political solution, Russia must end the obstruction of this process."