The United Nations wants to convene the committee as a next step in efforts to find a political solution to end the war in Syria, but there has been no agreement so far on who should be on it.
A Syrian Foreign Ministry statement said significant progress had been made in the talks, but also reiterated its previous stance that the constitutional committee should be a purely Syrian affair.
Pedersen also called for a return to stability in Idlib province, where a government offensive has targeted the last major rebel stronghold, and said a truce there agreed last year between Russia and Turkey should come back into force.
Russia is along with Iran the main supporter of President Bashar al-Assad, who now controls most of the country, while Turkey backs some of the rebel groups in Idlib and adjacent areas.
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