Russian President Vladimir Putin has said his Iranian and Turkish counterparts have supported a proposal to hold a "Syrian people's congress" that will bring together government and opposition figures.

The Russian leader on Wednesday hosted Hassan Rouhani and Recep Tayyip Erdogan, just as some Syrian opposition groups met in Riyadh, Saudi Arabia's capital, before United Nations-sponsored talks in Geneva.

"The congress will look at the key questions on Syria's national agenda," Putin told reporters at the summit in the Black Sea resort of Sochi, sitting alongside Rouhani and Erdogan.

"First of all, that is the drawing-up of a framework for the future structure of the state, the adoption of a new constitution, and, on the basis of that, the holding of elections under United Nations supervision."

The congress is expected to also take place in Sochi before the next round of Geneva talks on November 28. However, no details about the exact date or who will be invited to attend were released.

In a joint statement, the three leaders underlined the need for all warring sides to release all prisoners and hostages, hand over bodies and create the conditions for a lasting truce.

They also urged the international community to provide humanitarian aid, clear Syrian territory of mines and restore infrastructure wrecked by the long-running conflict.

Now in its seventh year, the war in Syria has killed hundreds of thousands of people and displaced more than 12 million.

Iran is a major ally of Syrian President Bashar al-Assad, who made a surprise visit to Sochi on Monday, while Turkey has backed some of his opponents.

The three countries, however, have joined forces to sponsor talks between the government and the opposition in Kazakhstan's capital, Astana, as well as act as guarantors for the establishment of four "de-escalation zones" across Syria.

Putin on Wednesday said he believed a "new stage" had been reached in the Syria crisis.

He warned, however, that achieving a political solution would require compromises from all sides.

Al Jazeera's Rory Challands, reporting from Sochi, said that Erdogan had, until now, appeared reluctant to back Putin's idea for a congress of a national dialogue.

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"Essentially, he is concerned that Kurdish groups are being invited, which he considers to be terrorist organisations," he said.

"Here, they all gave their support to it, but I still feel that Erdogan is a little bit lukewarm about it."

An interesting development at the leaders' press conference, Challands said, was a suggestion by Rouhani that the planned congress could prepare the ground for a new constitution for Syria.

"Some people have said that what Putin is trying to do with the Congress idea is bridge the gap between the Astana talks, which are about ceasefires and technical matters, and the much-stalled Geneva talks, which are about a grand political settlement," said Challands.

"This congress would sit in the middle, but it suggests that Putin is trying to shape the future of Constitution of Syria ahead of any future Geneva talks," he added.