Key international players in Syria's civil war gathered in Kazakhstan Wednesday for a fresh round of talks aimed at pushing a Russian-led plan to ease fighting on the ground.

Representatives from regime backers Russia, Iran and rebel supporter Turkey held "talks on an expert level" to lay the groundwork ahead of the start of two days of meetings that will include the Syrian regime and opposition representatives, Kazakhstan's foreign ministry said.

The talks in Astana are the sixth round of negotiations Moscow has spearheaded since the start of the year as it seeks to pacify Syria after its game-changing intervention on the side of leader Bashar al-Assad.

This time around the sides are looking to nail down details of a proposed "de-escalation" zone in the northern Idlib province, after Moscow ploughed on with setting up three other safe areas around the country in a move that has seen violence drop.

There remain major disagreements over which force will be sent to police the zone covering rebel-held Idlib -- on Syria's northern border with Turkey -- as Ankara and Tehran jockey for influence.

Russia has so far deployed military police to patrol the boundaries of three zones agreed in the south, in Eastern Ghouta near Damascus, and in part of the central Homs province.

Yehya al-Aridi, an advisor to the Syrian opposition, confirmed the rebel delegation would arrive in Astana on Wednesday evening, headed by Ahmed Berri, chief of staff of the Free Syrian Army.

"The aim of our participation in this round is to strengthen the de-escalation zones in Syria, in Eastern Ghouta, the south, and north of Homs, and to discuss many violations that took place since the ceasefire agreement was signed in Ankara at the end of the last year," al-Aridi told AFP.

A Syrian pro-regime newspaper Al-Watan reported that the government's delegation, led by the country's United Nations envoy Bashar al-Jaafari, had arrived in Astana early on Wednesday.

The de-escalation zone in the south of Syria has also proved sensitive as it lies close to the borders of US-allies Israel and Jordan, and Washington is keen to make sure that Iran and its proxy forces are kept out.

The State Department confirmed that Acting Assistant Secretary of State David Satterfield will travel to Astana Thursday as an observer.

"Iran's activities in Syria and unquestioning support for the Assad regime have perpetuated the conflict and increased the suffering of ordinary Syrians," the State Department said in a statement.