Secretary of State John Kerry said Thursday that prospects are uncertain at best for this weekend's Syria peace talks. But he says the diplomatic effort to end the war cannot be abandoned. Kerry said that although the elements necessary to forge peace may not exist, "time and turbulence" can create new possibilities that must be explored. However, he acknowledged that participants had to bridge vast differences to succeed, saying those involved are not on "the threshold" of an agreement. 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 nonstarter," 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." Meanwhile, Russia accused the United States of hijacking preparatory talks for the weekend meeting. The charge was denied by U.S. officials, who expressed surprise that Moscow did not show up. The dispute appeared to focus on the leading role the U.S. had assumed in the talks prior to the main Syria meeting Saturday, and was significant in reflecting the tensions between the two main players whose diplomatic muscle is key to hopes of progress. Kerry Saturday is set to join Russian Foreign Minister Sergey Lavrov and counterparts from other nations that include adversaries Iran and Saudi Arabia. Lavrov's spokeswoman, Maria Zakharova, said in Moscow that the Russian Foreign Ministry was informed about the preparatory meetings in an email from the U.S. Embassy, rather than through usual communication channels. She said Russia was not consulted and the initiative caught them by surprise. Russia sees this as an attempt to divide participants in the Vienna talks into "those who are leading and those who are being led," Zakharova said. "We cannot accept such rules of the game." She also complained that nations including Iran, Iraq, Lebanon, Italy and the EU – all participants in Saturday's meeting – were not included in the working groups. That was denied by U.S. and EU officials. One U.S. official, who demanded anonymity because he was not authorized to discuss the diplomacy, acknowledged that the U.S. had assumed leadership of the working groups set up by Staffan de Mistura, the U.N. envoy for Syria. But he said that – as Russia was a member of the working group meeting Thursday on identifying legitimate opposition groups – Washington had every reason to expect Russia to show up.