The UN envoy for Syria, Staffan de Mistura, said on Wednesday he will step down at the end of November for family reasons, after more than four years in the key post.

His resignation comes at a time as the Syrian government has retaken most of the country and a political deal remains elusive.

De Mistura is the longest serving of three UN mediators during the more-than-seven-year Syrian conflict. His predecessors – former UN Secretary-General Kofi Annan and veteran Algerian diplomat Lakhdar Brahimi – both quit in frustration over a global stalemate on how to end the war.

However, his successor faces a tough battle to negotiate a political deal, which the West has said is needed to unlock its reconstruction support and to encourage the bulk of the millions of refugees in Europe and the Middle East to return.

When asked if he was upset at not being able to achieve a breakthrough, de Mistura told reporters: "One month can be a century in politics.... I have always been an optimist."

Diplomats said possible replacements for de Mistura included UN Iraq envoy Jan Kubis, former Algerian foreign minister Ramtane Lamamra, UN Middle East envoy Nickolay Mladenov and Norway's Ambassador to China Geir Pedersen.

During his final weeks in the job, de Mistura told the Security Council that Secretary-General Antonio Guterres had instructed him to "verify once and for all" whether a credible, balanced constitutional committee could be convened.

Participants at a Syrian peace conference in Russia in January had agreed to form a committee to rewrite the Syrian constitution that will be made up of 150 people, with a third chosen by the government, a third by opposition groups and a third by the United Nations.

De Mistura said creation of the panel had been delayed as "questions continue to be raised, mainly by the Syrian government, over the composition" of the UN list.

He said he would visit Damascus next week and hoped to win government approval for a "credible and inclusive third list."