Russian-backed peace talks on Syria in Astana have been pushed back, Kazakhstan's foreign ministry said Thursday, as key players wrangle over the future of fragile safe zones in the war-torn country.

Moscow earlier said that it was calling a fresh round of negotiations in the Kazakh capital for June 12-13 to try to shore up four "de-escalation zones" agreed in May between the co-sponsors of the talks Russia, Iran and Turkey.

Kazakhstan's foreign ministry said it had been told representatives from the three powers would meet in their separate capitals in the "upcoming days and weeks".

"The sponsor countries intend to inform Kazakhstan in the future about the timeframe, level, attendees and agenda they agree for the next round of Astana talks," ministry spokesman Anuar Zhainakov was quoted as saying by Russia's RIA Novosti.

There was no immediate confirmation from Moscow, Ankara or Tehran on any new date for talks.

At the last round of negotiations in May attended by Syrian government and rebel delegations, regime backers Moscow and Tehran and rebel supporter Ankara agreed to establish four "de-escalation zones" to ease fighting in opposition areas.

The zones -- where aerial bombardments were supposed to stop -- have ushered in a marked decrease in fighting on the ground, but there remain key outstanding issues to negotiate.

Despite pledging to agree on definitive boundaries by June 4, the sponsors are still arguing over the outlines and which countries should send in forces to police the safe zones.

Moscow has spearheaded the Astana talks since the start of the year as it tries to turn its game-changing military intervention on the ground into a negotiated settlement.

The tetchy negotiations -- seen as a complement to broader UN-backed talks in Geneva -- have involved armed rebels and government officials and have focused mainly on military issues.