Staffan de Mistura has been frustrated that Syria is resisting the creation of a potentially vital committee

Staffan de Mistura, the outgoing UN special envoy for Syria, will today plead with Russia, Turkey and European powers to use the next month to force the Syrian regime to stop blocking the establishment of a committee to draw up a new constitution that could be key to ending the seven-year civil war.

Syria agreed with Russia at the beginning of the year to co-operate with the committee but has reneged on the plan as it eyes a non-negotiated total military victory.

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The committee had been seen as the best hope for a peaceful resolution to the war, but western powers believe the Syrian regime, victorious on the battlefield, is now being allowed by Russia to obstruct the plan.

Western countries believe that if no progress is made towards setting up the committee, Syria is likely to restart its efforts to recapture the province of Idlib where a fragile ceasefire had prevented an attack on the 3 million-strong province.

De Mistura told the UN security council that the Syrian foreign ministry refused on Wednesday to accept the UN had any role in setting up the committee and raised objections to the names of the independent members the UN had proposed to include.

The independents, including women, ethnic groups and civil society, are due to represent a third of the committee alongside a third for the Syrian government and a third for the opposition groups.

The Russian president, Vladimir Putin, will meet today in Istanbul with De Mistura, along with the Turkish president, Recep Tayyip Erdoğan, the German chancellor Angela Merkel, and the French president Emmanuel Macron.

Russia initially proposed the committee at a meeting convened amidst fanfare in the Black Sea resort of Sochi in January.

Moscow seems reluctant to acknowledge the role that was given at Sochi to the UN in drawing up the committee’s composition.

Many in the west and some UN officials feel they have been played by Russia as the Syrian government regained territory this year from the opposition. De Mistura recently said he was standing down as UN special envoy after four years for personal reasons, but his departure reflects his frustration at the UN’s lack of leverage in ending the war.

Speaking at a briefing of the UN security council in New York, the French envoy to the UN, François Delattre, called for the committee to be created immediately, pointing out the idea had been a Russian diplomatic initiative and claiming the Syrian regime could not be allowed “to ruin our collective efforts without which peace is unachievable”.

He said the coming weeks could be crucial and the risk of an escalation of the conflict could not be ruled out.

America’s deputy envoy, Jonathan Cohen, said “further obstruction on the committee’s formation is unacceptable. Further delay risks squandering the window of opportunity provided by the Turkish and Russian de-militarised zone that has succeeded in preventing further bloodshed in Idlib and beyond.

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“Parties have been consulted. No one should doubt that the UN special envoy has the mandate to move forward to establish the constitutional committee. Any list proposed by anyone must have the approval of the special envoy, not just in its membership, but in how it is compiled.”

Karen Pierce, the UK envoy to the UN, said she found it incredible that it was necessary to rehearse for the benefit of the Syrian government why the UN was involved in trying to settle the war.

“Either Russia has given the UN 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 said.

Vasily Nebenzya, the Russian envoy to the UN, said there are no grounds for artificial deadlines and accused the west of moralising against Russia.