Russian President Vladimir Putin said on Wednesday a "new stage" had been reached in the Syria crisis but achieving a political solution would require compromises from all sides, including the Syrian government.

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A three-way summit in Sochi on Wednesday between the leaders of Russia, Turkey and Iran could produce decisive steps towards ending the bloodshed in Syria, Turkish President Recep Tayyip Erdogan said at the start of their talks.

Putin announced he and his counterparts at Sochi, Erdogan and Iran's president Hassan Rohani, supported the convocation of a Syrian peoples' congress as one of the first steps to establish inclusive dialogue in the war-ravaged country. Putin said the three leaders had instructed their diplomats, security and defense bodies to work on the composition and date of the congress.

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Syria's leadership is committed to the peace process, constitutional reform and free elections, Putin said. The three presidents agreed to step up efforts to finish off "terrorist" groups in Syria, he said.

The Wednesday summit, hosted by Russian President Vladimir Putin, is a rare occasion bringing together the leaders of Russia and Iran – who back Syria's President Bashar Assad – around a table with Erdogan, who has supported Assad's opponents.

Russia, Iran and Turkey have helped Syria to maintain stability and avoid being taken over by international terrorists, which would have been a "humanitarian catastrophe," Putin said in comments carried by state media.

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"We can say with certainty that we have reached a new stage, opening up the possibility to launch a real political process towards a peace deal," Putin told the gathering.

"Compromises and concessions will be needed on all sides ... including (from) the Syrian government," Putin said.

"The point we have reached is important, but not enough," Erdogan told the gathering, also attended by military commanders and foreign ministers from the three countries.

"It is critical for all parties to contribute to a permanent and acceptable political solution for the people of Syria," he said. "This summit is aimed at results, I believe critical decisions will be taken."

Iran's Rohani used his remarks at the summit to rail against the presence of foreign forces in Syria, an apparent reference to the United States and Tehran's arch regional rival Saudi Arabia, which alongside Turkey have backed Assad's foes.

"There is no excuse for the presence of foreign troops in Syria without the approval of its legitimate government," Rohani said.

"The Syrian nation will not allow any interference of foreigners in their state affairs and will confront any move that harms Syria's integrity, independence and unity," he said.

Iran's military is also present in Syria, alongside Russian troops and Hezbollah, the pro-Iran Lebanese militia. They say that does not amount to foreign interference because they are in Syria at Assad's invitation.

As a prelude to the summit, Putin earlier this week hosted Assad at his residence in Sochi. It was the only time the Syrian leader is known to have left Syria since his last visit to Russia, two years ago.

Putin also made telephone calls in the past 24 hours to other leaders with influence in Syria, including Israeli Prime Minister Benjamin Netanyahu, U.S. President Donald Trump and Saudi Arabia's King Salman, as part of Moscow's drive to build an international consensus over a peace deal to end the six-year conflict.