UNITED NATIONS -- The UN special envoy for Syria announced on Wednesday that he will step down at the end of November but hopes to put together a constitutional committee for the war-torn country by then.

After serving for four years and four months on the demanding job, Special Envoy of the Secretary-General for Syria Staffan de Mistura said he decided to quit for "personal reasons."

He told UN Security Council that the Syrian government has invited him to Damascus next week, where he plans to engage them on the composition of the constitutional committee.

That plan to draft a new Syrian constitution has been under discussion since January when it was announced at a conference organized by Russia, a key Syrian ally, in the Black Sea resort of Sochi. But it has since been hampered by disagreement over the formation of the committee to draft the charter.

The Syrian government's and the opposition's lists for the committee, each comprising 50 names, "are not in question," said de Mistura, adding however, objection remains, mainly from the government, over a third list that the UN is tasked to form to include civil society representatives, religious and tribal leaders, experts and women.

The UN envoy expressed his hope to "be in a position" to issue invitations to convene the constitutional committee, hopefully during the month of November.

"The UN has done all it can to find a way to convene a credible and balanced constitutional committee," he said, pledging to do more and in an accelerated way in the forthcoming month, taking advantage of the "Idlib window of opportunity."

In September, Russia and Turkey, key stakeholders in Syria, agreed to set up a demilitarized zone between rebel and government fighters in the northwestern province of Idlib, staving off a major government offensive on the area, which the UN had warned could cause a humanitarian catastrophe.

De Mistura is the third UN envoy for Syria since the outbreak of the conflict in 2011, after former UN chief Kofi Annan and Algerian diplomat Lakhdar Brahimi stepped down.

At a press encounter Wednesday, asked if he still believes in a political solution to the crisis, de Mistura said it is still the key as "the alternative will be territorial gains and no sustainable peace."

"Our assessment is that if there is a political will, there is no reason -- no reason whatsoever -- for the constitutional committee not to be able to be convened during the month of November," said de Mistura at the encounter.

The main reason for the delay, he said, is the Syrian government's difficulties to accept the final list of committee members prepared by the UN.

He said he will try to get the Syrian government on board when he travels to Damascus next week.

De Mistura said he will report back in November to UN Secretary-General Antonio Guterres and the Security Council for them to draw conclusions on this issue. He expressed the hope that the conclusions will help his successor start from a clear and clean ground.