U.S. Secretary of State John Kerry will meet his Iranian counterpart in Vienna on Thursday ahead of a fresh round of talks on ending the Syrian conflict.

U.S. Secretary of State John Kerry will meet his Iranian counterpart in Vienna on Thursday ahead of a fresh round of talks on ending the Syrian conflict, a senior State Department official said.

Tehran will join the talks on Friday in what will be the first time it has participated in international negotiations aimed at resolving the bloodshed as key nations backing Syria’s warring sides seek to thrash out differences over the future of President Bashar al-Assad.

The inclusion of Iran — a key Assad backer — marks a crucial shift after it was excluded from earlier talks, largely due to opposition from the United States and Saudi Arabia.

Iranian Foreign Minister Mohammad Javad Zarif was also to meet with his Russian counterpart Sergei Lavrov on Thursday, according to the Russian news wire Interfax.

The talks come in the context of two days of diplomatic efforts involving all the major international players in the same room for the first time.

On Thursday evening, Russia — which backs Assad — will hold talks with the United States, Saudi Arabia and Turkey, all of whom back rebels seeking to overthrow the Syrian leader.

It will be the second time in less than a week that the four envoys have met on Syria.

But Friday’s talks will be a wider gathering of regional players which will see Zarif joining representatives from Britain, France, Germany, Italy, Egypt, Lebanon and the European Union.

So far there has been no mention of either the Syrian government or the opposition joining.

The sides remain bitterly divided over the question of Assad, and an earlier round of talks on October 23 ended inconclusively.

On one side, Russia and Iran are backing Assad’s forces on the ground and say Damascus must be helped to defeat “terrorism” before a political process can take shape.

On the other, the US and its key regional allies Turkey and Saudi Arabia are supporting groups fighting Assad and insist he must go.

Around a quarter of a million people have died in the Syrian conflict over the past four and a half years.