Russia, Iran, and Turkey have agreed at peace talks in Kazakhstan to place observers on the border of a de-escalation zone in Syria for at least six months.

The announcement comes as part of a broader plan under which Moscow, Tehran, and Ankara will create four zones in different parts of Syria, a move some critics called a de facto partitioning of the war-torn country.

The zones will include, either fully or partly, Eastern Ghouta in the northern Damascus countryside, and the provinces of Idlib, Homs, Latakia, Aleppo, and Hama. The six-month term could be extended in the future.

Russian negotiator Aleksandr Lavrentyev said the trio will each send about 500 observers to Idlib, and that the Russians will serve as a military police force.

"Observers from these three countries will be deployed at check and observation points in safe zones that form the borders of the de-escalation zone," Turkey's Foreign Ministry said in a statement after two days of talks in Kazakhstan's capital, Astana.

Russia, Turkey, and Iran are sponsors of the Astana talks on Syria, which are separate from United Nations-sponsored talks in Geneva.

The three countries signed a memorandum in May that called for the creation of the four de-escalation zones.

Russia has moved to establish three of them and there has since been a drop in violence between combatants.

But differences over the borders of the fourth proposed de-escalation zone have prevented the signing of a formal agreement on the creation of all four zones.

The U.S. State Department has said that Washington "remains concerned with Iran's involvement as a so-called 'guarantor' of the Astana process."

The U.S. government is concerned about calls for Iranian forces to also be deployed as cease-fire monitors.

It says Iran's "activities in Syria and unquestioning support" for Syrian President Bashar al-Assad's government "have perpetuated the conflict and increased the suffering of ordinary Syrians."

Some Syrian opposition fighters also have rejected the idea of Iranian forces being given a role as cease-fire monitors, saying they are not neutral forces.

With reporting by Reuters, Anadolu, Yeni Safak, AP, AFP, Kazinform, TASS, Izvestia, Interfax, and RIA Novosti