Russian President Vladimir Putin on Monday made a shock announcement that he had ordered the withdrawal of “the main part” of Russia’s forces after talks with long-standing ally, Syrian President Bashar al-Assad.

“The announcement by President Putin on the very day of the beginning of this round of intra-Syrian talks in Geneva is a significant development,” Staffan de Mistura said in a statement read by his spokesman Ahmad Fawzi.

“We hope (this) will have a positive impact on the progress of the negotiations in Geneva aimed at achieving a political solution of the Syrian conflict and a peaceful political transition in the country,” he added.

Fawzi told reporters that de Mistura had been informed of the Russian decision on Monday evening, shortly before he briefed the UN Security Council via video-link about progress made on the first day of a new round of Syria peace talks in the Swiss city.

Putin’s announcement came ahead of the fifth anniversary of the start of Syria’s brutal conflict, which has left more than 270,000 dead and forced millions to flee their homes.

“Today is an important day,” Fawzi said.

He pointed out that it was not only the fifth anniversary of the start of Syria’s “tragedy” but also the 18th day of a partial truce, which he said was “fragile, vulnerable, but holding by and large.”

Western leaders meanwhile reacted cautiously to the Russian announcement, with Moscow yet to specify a timeframe for the withdrawal and set to maintain its air and naval base and advanced air defence systems in Syria.

And hopes for a breakthrough at the Geneva talks remained remote, with both sides locked in a bitter dispute over Assad’s future.