The UN peace envoy for Syria on Tuesday urged world powers to build on the “momentum” of new international talks and come up with a political process to end the four-year war.

“The momentum in Vienna needs to not be missed,” Staffan de Mistura told reporters after briefing the UN Security Council, as some 20 countries and international bodies plan to meet in the Austrian capital Saturday to try to push forward a peace plan for Syria that would include a ceasefire between Bashar Al-Assad’s regime and some opposition groups.

The envoy said the meetings must “bring some deliverables to the Syrian people. One of them should be reduction of violence.”

Three working groups will begin meetings on Wednesday to address differences over terrorism, the opposition and the humanitarian crisis from the war that has left 250,000 dead.

Russia and Iran, which back Assad, are at odds with the United States and its European and Arab allies over which groups should be branded as “terrorists” and which ones qualify as members of the opposition.

The debate will determine who will play a role in the political process.

While the United Nations has branded the Islamic State group, the Al-Qaeda-linked Al-Nusra Front and other smaller groups as terrorists, the envoy said it was up to the countries involved to come up with a shared view.

“My job is to make sure that big countries like the Russian Federation, Saudi Arabia and Iran come around the table and come up with a political process,” de Mistura said.

“It’s time for those countries to pick up those challenges,” he added.

During his closed-door meeting with the council, de Mistura held out the prospect of a “concrete outcome” to the Vienna talks, diplomats said.

The UN envoy’s comments came amid reports that Russia had drafted a plan for Syria which would see the Syrian government and opposition to agree on launching a constitutional reform process of up to 18 months, followed by early presidential elections.

Russian Deputy Foreign Minister Mikhail Bogdanov, however, denied Moscow had produced a document, saying his country had only come up with “different ideas and proposals” which did not constitute a solid plan.

AFP contributed to this report.