Syria wants the United Nations to simply facilitate efforts to rewrite the country's constitution and not choose a third of the committee that will undertake the job, outgoing U.N. Syria envoy Staffan de Mistura told the Security Council on Friday.De Mistura briefed the 15-member body after meeting Syrian Foreign Minister Walid al-Moualem in Damascus this week. De Mistura wants to convene a constitutional committee in November before he steps down at the end of the month, but he has accused the Syrian government of delaying the process."We do have a serious challenge, let's be frank. It is my intention to spare no efforts to address this challenge during the forthcoming weeks," de Mistura told the council.Participants at a Syrian peace conference in Russia in January agreed to form the 150-member committee to rewrite the Syrian constitution with a third chosen by the government, a third by opposition groups and a third by the United Nations."Minister Moualem did not accept a role for the U.N. in general in identifying or selecting the ... third list," de Mistura said. "He underlined that the Syrian constitution is a highly sensitive matter of national sovereignty."He said Moualem indicated that Syria and Russia had recently agreed that the third list of participants should be decided by Syria, Russia, Turkey and Iran - the three countries pushing separate Syria political talks known as the Astana process - and then given to the United Nations for facilitation.British U.N. Ambassador Karen Pierce questioned Russia's motives in trying to negotiate a political solution given Syria's efforts to stall a constitutional committee meeting."Either Russia has given the U.N. and this council assurances it has proved too weak to deliver on, or it was all a cynical smokescreen designed to divert attention and energy while Russia, Syria and Iran prosecuted the military campaign," she told the council.Russian U.N. Ambassador Vassily Nebenzia said there were "no grounds for establishing artificial deadlines for the establishment of the constitutional committee."De Mistura said he would attend a summit with the leaders of Russia, Turkey, France and Germany in Istanbul on Saturday.De Mistura's 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.Diplomats say Norway's Ambassador to China Geir Pedersen has emerged as a front-runner to become the fourth U.N. Syria envoy during the more than seven-year-old war.