BEIRUT (Reuters) - Russia’s planned peace talks among Syrian groups will only succeed if the opposition ends its fight against the government, a senior adviser to President Bashar al-Assad said on Thursday.

FILE PHOTO - Bouthaina Shaaban, advisor to Syrian President Bashar al-Assad speaks to the media after a meeting at the Geneva Conference on Syria at the United Nations European headquarters in Geneva January 30, 2014. REUTERS/Denis Balibouse

“The success of the congress depends on the various opposition groups realizing that the time has come to stop the violence, lay down their weapons and engage in a national dialogue,” said Bouthaina Shaaban in comments to a Russian news agency carried by Syrian state media.

Russian President Vladimir Putin met Assad as well as the leaders of Iran and Turkey in the Black Sea resort of Sochi this week in a diplomatic push to prepare for a congress between Syria’s government and opposition.

For years, Western and Arab countries backed the opposition demand that Assad quit power, but since Russia’s 2015 entry into the war, his government has won back major cities and now looks militarily unassailable.

Large parts of northwest and southwest Syria remain in rebel hands as well as a pocket near Damascus, and Kurdish-led groups hold much of the northeast.

Opposition groups who met in Saudi Arabia on Wednesday will stick to their demand that Assad leave power as part of any political transition, the Saudi-owned al-Arabiya channel reported.

But after the Syrian army and its allies recaptured Albu Kamal, Islamic State’s last Syrian town, Putin said the military campaign in Syria was winding down.

Russia has not said when the Syrian congress, also planned to take place in Sochi, will take place or who will be invited.

The congress will involve drawing up a framework for the future structure of the Syrian state, adopting a new constitution and holding elections under United Nations supervision, Putin said.

After their meeting on Wednesday, Putin, Turkish President Tayyip Erdogan and Iranian President Hassan Rouhani called on the Syrian government and moderate opposition to “participate constructively”.

Shaaban said the government is ready for dialogue with those who believe in a political solution, adding that the opposition’s desire - or even ability - to engage in a real political operation has not yet been made clear.

Syria’s six-year-old civil war has killed hundreds of thousands, pushed millions to flee in the worst refugee crisis since World War Two and embroiled regional and world powers.