VIENNA — The Syrian government’s biggest international backers and opponents plunged into negotiations Friday over a long-sought compromise to help end a 4 and-a-half year civil war and potentially ease President Bashar Assad out of power. U.S. Secretary of State John Kerry said he was hopeful of finding a path forward.

In Austria’s capital, Kerry was negotiating with foreign ministers and senior representatives of 18 other countries. They included Iran for the first time, making it the broadest gathering of nations yet to discuss Syria’s future. Another key supporter of Assad, Russia, was present, along with many of the most influential Arab and European allies of the United States.

Several participants argued that the talks themselves were a sign of progress. But with no end to the war in sight, there was pressure on all sides to begin chipping away at a “political transition” plan that might convince Assad’s government and the vast array of armed rebel groups to stop fighting and allow world powers to focus on their shared commitment to defeat the Islamic State.