By Michelle Nichols
UNITED NATIONS (Reuters) - U.N. Syria envoy Staffan de Mistura said on Wednesday he would step down at the end of November for family reasons, quitting as the Syrian government - backed by Iran and Russia - has retaken most of the country and a political deal remains elusive.
Despite the advances by Syrian President Bashar al-Assad, de Mistura said a political solution was still the key as "the alternative will be territorial gains and no sustainable peace."
De Mistura is the longest serving of three U.N. mediators during the more-than-seven-year Syrian conflict. His predecessors - former U.N. 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 - who has been in the job for more than four years - told reporters: "One month can be a century in politics. ... I have always been an optimist."
Diplomats said possible replacements for de Mistura included U.N. Iraq envoy Jan Kubis, former Algerian foreign minister Ramtane Lamamra, U.N. 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 U.N. list. He said he would visit Damascus next week and hoped to win government approval for a "credible and inclusive third list."
"If there is a political will, there is no reason for the constitutional committee not to be convened in November," he told the Security Council.
The 15-member U.N. Security Council has long been deadlocked with Western powers pitted against Syrian ally Russia, backed by China.
French U.N. Ambassador Francois Delattre accused the Syrian government of obstruction, "which ultimately shows us that it does not want to participate in the diplomatic efforts underway, while its allies are either unable or unwilling to convince it to put an end to its intransigence."
Russian U.N. Ambassador Vassily Nebenzia said Moscow wanted the constitutional committee to be formed as soon as possible but that "setting artificial deadlines in this case would be counterproductive."
"We need to be patient," Nebenzia said.
(Reporting by Michelle Nichols; Writing by Lisa Lambert; Editing by Bernadette Baum, Susan Thomas and Jonathan Oatis)