Officials from Turkey, Russian and Iran on Monday agreed to the mutual release of some Syrian detainees from both the regime and opposition sides.

According to participants attending the pre-meeting in the Russian city of Sochi ahead of a 10th round of Syria peace talks in Astana, Kazakhstan, as part of confidence-building measures, the sides will mutually release a limited number of detainees that they held.

No further details were provided by the participants -- who asked not to be named due to restrictions on speaking to the media -- on how many detainees will be released or from what region.

The 10th round of Syria peace talks will start in Astana, Kazakhstan on Tuesday, which will address the latest situation in de-escalation zones in Idlib, Syria, the return of refugees, the release of prisoners, and the formation of a committee that will rewrite Syria's Constitution.

During the 9th Astana meeting held on May 14-15, all parties agreed to maintain the de-escalation zones and the cease-fire regime.

However, the Assad forces and their allies took control of the de-escalation zones in the Homs region, Daraa, and Quintieri following a heavy offensive.

Last week, the regime began a military buildup in southwestern Idlib in a bid to attack Syria’s northwestern Turkmen Mountain region, which is located within the de-escalation zones.

Idlib is set to be a top issue during the Astana meeting, as the Turkish delegation is expected to stress the importance of maintaining the cease-fire regime in the region.

Separately, Russia during the meeting is expected for the first time to bring up the issue of the return of refugees and displaced persons.

Russian officials have recently visited Turkey, Jordan, and Lebanon, which host the majority of Syria's refugees.

UN Special Envoy for Syria Staffan de Mistura is also expected to meet with the Turkish, Russian and Iranian officials on the formation of a constitutional committee.

The regime and opposition had each submitted to de Mistura a list of 50 candidates to represent them on the committee.