World leaders will converge in Vienna Saturday in the latest attempt to hammer out differences on proposed solutions to the turmoil in Syria. At the new round of talks, the U.S. and its allies -- who have been insistent that Syrian President Bashar Assad must leave office for the peace process to progress -- would seek to work out differences with Russia and Iran, who say Syrian voters should have the right to decide his fate.

“There are a number of difficult issues of which the future of Bashar al-Assad is ‎probably the most difficult and that will certainly be an important subject tomorrow,” British Foreign Secretary Philip Hammond, who will attend the talks in Vienna, reportedly said Friday.

Earlier in the day, speaking at a news conference in Prague, Hammond had reiterated that Assad, who has clung to power despite increasing international pressure and intensifying airstrikes in Syria, would have to relinquish power as part of the political transition in the country. However, he added, “he [Assad] may play a part in that up to a point of departure.”

The talks are the second in two weeks to bring together world powers to thrash out the prospects of a political transition. The previous round, which concluded late last month, only served to highlight that on the issue of Assad’s political future, the U.S. and Russia, as well as Saudi Arabia and Iran -- which will also attend Saturday’s negotiations -- remain far apart.

“The walls of mistrust within Syria, within the region, within the international community are thick and they are high,” U.S. Secretary of State John Kerry, who arrived at the Austrian capital Friday, reportedly said. “I cannot say this afternoon that we are on the threshold of a comprehensive agreement.”

The talks in Vienna -- which will be attended by 17 nations, along with representatives from the United Nations, the European Union and the Arab League -- are also expected to focus on identifying Syria’s moderate opposition groups, as well as those that are considered terrorist entities. Acknowledging the difficulty in doing so, Kerry said: “There are bad guys all around and good guys who are not accustomed to working with each other.”

The multi-pronged conflict in Syria has pitted Assad’s forces against the so-called moderate rebels -- backed by the U.S. and its Western and Arab allies. Both these factions are also fighting the Islamic State group, which, despite recent setbacks in Iraq, still retains control over vast swathes of northern Syria.

Estimates suggest that the conflict, now in its fifth year, has so far led the deaths of nearly 300,000 people and forced nearly four million people to flee the country.