UN special envoy Staffan de Mistura has been working since January on forming a group of 150 members to thrash out a new constitution, seen as a first step to winding down a conflict that has cost more than 360,000 lives. Syrian regime artillery fire killed seven civilians in Idlib. BEIRUT: A United Nations plan to end the seven-year civil war in Syria has run aground after Damascus blocked the world body"s proposal for a committee to draft a new constitution, provoking anger among western powers. UN special envoy Staffan de Mistura has been working since January on forming a group of 150 members to thrash out a new constitution, seen as a first step to winding down a conflict that has cost more than 360,000 lives. Under the UN plan, the Syrian regime would choose 50 of the committee members, the Syrian opposition another 50 and the UN would nominate the final 50, composed of representatives of civil society and technical experts. De Mistura, a veteran Italian-Swedish diplomat due to step down next month, said Friday that Foreign Minister Walid Muallem rejected the last list of UN-proposed names and suggested his own method to the final 50 members during talks in Damascus this week. "Walid Muallem didn"t accept a role for the UN in identifying or selecting a third list," the envoy told the UN Security Council by video conference during an emergency session called by the United States. "Rather, Mr Muallem indicated that the governments of Syria and Russia had agreed recently that the three Astana guarantors (Iran, Russia and Turkey) and the Syrian government would in consultations among them prepare a proposal as regards the third list." De Mistura said withdrawing the UN list was only possible "if there was an agreement on a new credible, balanced and inclusive list" that complied with UN resolutions and commitments made in January talks in the Russian resort town of Sochi. Also on Friday, Syrian regime artillery fire killed seven civilians in Idlib, in the highest death toll since a deal last month to prevent a government assault on the province, a monitor said. Three children were among those killed in the country"s last major rebel bastion in the northwest of the country, the Britain-based Syrian Observatory for Human Rights said. Regime ally Russia and rebel backer Turkey agreed on September 17 to set up a buffer zone around the Idlib region, which includes the province of the same name and parts of adjacent provinces. The deal was intended to protect three million inhabitants in the region, more than half of which is held by the Tahrir Hayat al-Sham (HTS) alliance led by militants of Al-Qaeda"s former Syria affiliate. Friday"s deadly shelling hit Al-Rifa, an HTS-held village in the southeast of Idlib province. In the neighbouring province of Aleppo, regime fighters and rebels exchanged gunfire on the western outskirts of the provincial capital, the Observatory said. Extremists including HTS were to withdraw from the expected demilitarised zone under the Russian-Turkish deal, but did not do so by an October 15 deadline. After that date passed, shelling continued intermittently and escalated dramatically late Wednesday. Government rocket and artillery fire killed one girl in Kafr Hamra, a town in Aleppo province inside the planned buffer zone, the Observatory said. And rocket fire by both extremists and Turkish-backed rebels hit second city Aleppo, wounding 10 people. Both Russia and Turkey have said the truce deal remains on course despite the missed withdrawal deadline. The leaders of the two countries are to be joined by their French and German counterparts for a four-way summit on Syria in Istanbul on Saturday. Syria"s regime has insisted that the buffer zone deal is temporary and that Idlib would eventually revert to government control. On Friday, Syria"s UN envoy Bashar Jaafari repeated this in comments reported by state news agency SANA. "It is normal that the Syrian state fights terrorism in Idlib to rid its people of terrorism, and to extend its sovereignty over it," he said.