The Kremlin said Thursday Turkey's opposition to the participation of Kurdish militias in Syria's political process would not stand in the way of a peace "congress" Moscow is seeking to organise in the near future.

"We know that there are certain reservations on the part of our Turkish partners with regards to the forces they believe pose a threat to their national security," Kremlin spokesman Dmitry Peskov told reporters at the Black Sea resort of Sochi.

"But this does not mean that work will not be conducted. Intense expert work to agree and check the lists (of congress participants) lies ahead."

He said the congress would be convened "in the near future" but did not provide more details.

On Wednesday, President Vladimir Putin hosted the leaders of Iran and Turkey for a key trilateral summit aimed at finding a political settlement of Syria's six-year civil war.

The trio sought to show a united front, saying they hoped a Moscow-championed "congress" would bring together Syrian strongman Bashar al-Assad's forces and various opposition groups and reinvigorate a hobbled peace process.

But Turkish President Recep Tayyip Erdogan appeared to fume at the prospect of inviting the Kurdish group PYD and its armed wing, the Kurdish Popular Protection Units (YPG) fighting in Syria to take part in the peace conference.

Without referring to the Kurdish militias by name, Erdogan said at the Sochi summit: "We cannot consider a terrorist gang with blood on their hands a legitimate actor."

"The exclusion of terrorist elements that threaten Syria's political unity and territorial integrity, as well as our national security, will continue to be a priority for Turkey," Erdogan said.

Moscow, Ankara and Tehran are cooperating with increasing intensity on ending the civil war, even though Turkey backs the rebels, at odds with Russia and Iran.