The United States does not expect a major breakthrough at international talks in Vienna on how to end Syria's civil war, with participants divided over the fate of President Bashar al-Assad.
But US Secretary of State John Kerry believes enough common ground exists between the main protagonists Washington, Russia, Saudi Arabia and Iran to "develop a timetable for action based on interim steps."
The weekend talks bringing together 17 countries and three international bodies are the second round of negotiations between foreign actors with a stake in Syria's four-and-a-half-year war.
The aim is to agree on a structure for a political transition and to decide which of the Syrian government, rebel and opposition factions -- none of them yet represented at the talks -- will take part.
At the first round last month, Kerry and his Russian counterpart Sergei Lavrov "agreed to disagree" about Assad's role, allowing the talks to go forward but leaving their eventual success in doubt.
"I cannot say this afternoon that we are on the threshold of a comprehensive agreement, no. There remains a lot of work to do," Kerry said, in a speech to the US Institute of Peace, before leaving for Europe.
"The walls of mistrust within Syria, within the region, and within the international community, are thick and they are high," he warned.
Signup to our newsletter and follow us on Facebook and Twitter!
"But those walls will never be breached unless we make a concerted and creative effort to surmount them."
According to a leaked version of the latest draft Russian proposal, Moscow wants ally Assad to stay in office during an 18-month transitional period.
For Washington and its Western and Arab allies, this is impossible.
"Asking the opposition to trust Assad or to accept Assad's leadership is simply not a reasonable request. It's literally a non-starter," Kerry warned.
"On this point, I acknowledge that we are still working through with Russia and Iran the question of Assad and his role, this has not been settled."
Kerry repeated the US view that Assad's initial, bloody crackdown on pro-democracy protests had created room for the rise of the Islamic State jihadist group, and that only his departure can reunite Syrians.
French and British officials have suggested that the talks will try to agree a list of groups that can be considered part of the peace process and a list of "terrorists" to face military action.
Kerry did not go into detail on this possible avenue of progress, but said that while the powers would discuss the "possibility of a nationwide ceasefire" this would not apply to action against the Islamic State group.