MOSCOW — Russia’s push with Turkey and Iran to end the war in Syria enters a new phase on Monday with peace talks in Kazakhstan that leave the U.S. effectively on the sidelines.

Syrian officials and representatives of armed opposition groups will meet in the Kazakh capital, Astana, for negotiations hosted by the three powers. The talks follow a cease-fire in Syria brokered by Russia and Turkey late last month that continues mostly to hold, in contrast to earlier U.S.-Russian attempts. It’s not clear if the talks will be face to face.

The U.S. is now “marginal to the war” in Syria and its relative exclusion from the initiative will “not prevent the Turks and the Russians coming to and implementing an agreement,” said Faysal Itani, an analyst with the Atlantic Council in Washington.

Amid deteriorating relations with the Obama administration over a failed peace effort with the U.S. in September, Russia seized the diplomatic initiative in Syria after its forces helped President Bashar al-Assad to expel rebel fighters from Aleppo, the country’s largest city, last month. The taking of the city was a turning point in the six-year civil war, which has killed more than 300,000.

The Astana talks are part of a joint approach announced in Moscow last month by Russia, Turkey and Iran, the three powers with forces on the ground in Syria. While Russia and Iran support Assad’s regime, Turkey, a key backer of the armed groups opposing the Syrian leader, helped to negotiate the truce accord signed by seven Islamist groups representing 62,000 fighters and the Damascus government.

The agreement excluded Islamic State and the al-Qaeda affiliate in Syria because the UN Security Council has declared them terrorist organizations.

While the U.S. under President Obama was left out, Russian Foreign Minister Sergei Lavrov said last week that aides to new President Trump have been invited to Astana. The U.S. ambassador to Kazakhstan will attend the talks as an observer, State Department acting spokesman Mark Toner said in a statement on Saturday.

“The center of gravity in the Syrian conflict is moving away from great powers to regional players,” including Iran, Turkey and the other rebel backers Saudi Arabia and Qatar, said Andrey Kortunov, director general of the Russian International Affairs Council, a research group set up by the Kremlin.

“A lot will depend on whether there is a chance to bring all these players together and whether a country like Russia might play an instrumental role in that,” he said.

Underlining the role of the Astana meeting, UN Secretary-General Antonio Guterres asked his special envoy to Syria, Staffan de Mistura, who is due to resume talks next month in Geneva, to participate. Guterres views the Astana initiative as a “positive step,” his spokesman Stephane Dujarric said Thursday in a statement.

The involvement of Syrian armed groups at the meeting contrasts with past abortive rounds of UN-led talks, though their chief negotiator – a senior figure in the Army of Islam – also led the main opposition delegation in Geneva. Another major militant organization, Ahrar as-Sham, says it won’t be in Astana because of cease-fire violations by the government and its allies.

The talks, which are expected to last several days, aim to cement the truce and pave the way for political dialogue, Russian Foreign Ministry spokeswoman Maria Zakharova said Thursday.

“That would be a big achievement if it happened,” said Bassma Kodmani, a leader of the High Negotiations Council, the main Syrian opposition bloc. “Turkey and Russia are two decisive players on the ground, but there is one player – Iran – with which it’s much more difficult to know if there is any interest in a cessation of hostilities and a political process.”