Russia's President Vladimir Putin will host a trilateral summit with his Iranian and Turkish counterparts in the Russian Black Sea city of Sochi.
Thursday's meeting between Putin, Iran's Hassan Rouhani and Turkey's Recep Tayyip Erdogan will focus on the long-term settlement of the Syrian crisis, the Kremlin said in a statement on Monday.
But the three leaders will also discuss projects and coordination on the international arena. On the sidelines of the summit, the trio will hold individual meetings, according to the Kremlin.
The summit is the fourth of its kind since Putin, Rouhani and Erdogan first met in Sochi in 2017.
The three states refer to themselves as "guarantor countries", claiming they fight for securing peace in Syria. In December 2016, they brokered a ceasefire, setting up the so-called "de-escalation zones" in the war-torn country.
The Syria talks run in parallel to the Geneva talks organised by the United Nations.
But Russia distrusts the negotiations organised by the West. On Wednesday, Russia stayed away from a Middle East conference organised by the United States in Poland, a NATO member.
The Russian foreign ministry said in comments carried by the Russian news agency Interfax that Moscow views with concern "US attempts to impose unilateral geopolitical interests through initiatives presented as opinions of the entire international community".
Iran also criticised the US for seeking the withdrawal of the Iranian military out of Syria.
"The conspiracy of major states like the US will not affect our policies," Iranian President Hassan Rouhani said in comments carried by Russian state news agency, TASS.
Defence ministers meet
Thursday's Sochi talks were preceded by a meeting between Russia's Defence Minister Sergei Shoigu and his Turkish counterpart Hulusi Akar in the Turkish capital Ankara on Monday.
Shoigu and Akar said they needed to coordinate their next moves on Syria, solving a few remaining issues ahead of Thursday's summit.
Russia and Turkey share a strong opposition to the US military presence in Syria and are coordinating ahead of the planned withdrawal of Washington's troops.
Iran and Russia are close allies of Syrian President Bashar al-Assad, while Turkey supports the opposition.
Maria Zakharova, the spokeswoman of Russia's foreign ministry, said last week that Russia demanded Turkey do more to tackle rebel fighters in Syria's Idlib province and fulfil promises it made as part of a deal with Moscow last year.
Turkey and Russia brokered a deal in September to create a demilitarised zone in the northwest Idlib region that would be evacuated of all heavy weapons and opposition fighters.
The deal helped avert a government assault on the area, the last major bastion of Assad's opponents.
Speaking at a news conference in Moscow, Zakharova said the situation there was rapidly deteriorating and that rebels were trying to seize control of the entire de-escalation zone.
"Given the extremely difficult situation in the Idlib de-escalation zone, we expect our Turkish partners to activate their efforts to ultimately turn the tide and to fully carry out the obligations they took upon themselves," Zakharova was quoted as saying by the foreign ministry.