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The outgoing UN envoy for Syria has appealed to the country's warring sides to form a committee that would negotiate a new constitution as a way of bringing the Arab nation out of its protracted civil war.

Staffan de Mistura said there are disagreements over a "few names" of those who would be on that committee.

He said "agreement, particularly on the side of the government" of President Bashar al-Assad was needed.

De Mistura told reporters Friday that his Dec. 20 briefing to the UN Security Council could be his last.

He had originally planned to leave in November.

De Mistura appeared alongside China's special envoy Xie Xiaoyan and said he was seeking Chinese help to convince Syria's government that it's "worth it to make an effort."

The leaders of Russia, rebel backer Turkey, Germany, and France called in October for a committee to be formed by the end of the year to discuss a post-war constitution, "paving the way for free and fair elections" in Syria. 

The opposition has pushed for an entirely new constitution, but the regime has said it will only discuss altering the current one.

Assad's forces have notched up a series of victories against rebels and jihadists since Russia intervened militarily on its side in 2015, and now control almost two-thirds of the country.

Syria's war has killed more than 360,000 people and displaced millions since the war broke out in 2011 with the brutal repression of anti-government protests.