VIENNA (AP) � 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. A breakthrough wasn't immediately apparent.

As the Vienna talks were happening, Syrian opposition reported that a government missile barrage killed more than 40 in a Damascus suburb. The conflict has claimed more than 250,000 lives and uprooted more than 11 million people since 2011, leading to the growing terrorist threat of the Islamic State and sparking a refugee crisis throughout Europe.

"I am hopeful that we can find a way forward," Kerry told reporters, before adding: "It is very difficult."

Assad's fate was at the center of discussions. The U.S., Saudi Arabia and others have tempered their earlier calls for Assad's immediate ouster and now say he can remain in office for months as part of a transition if he agrees to resign at the end of the process. Russia and Iran are both providing Assad military assistance and say Syria's leadership shouldn't be dictated by outside forces.

Offering some hope, however, both countries have suggested greater flexibility in recent weeks. Western diplomats have spoken of various conversations with their Russian counterparts indicating that the Kremlin is not "wedded" to Assad maintaining control of the country. And senior Iranian diplomat Hossein Amir Abdollahain told The Guardian last week, "We are not working for Assad to stay in power forever as president."

But no one has provided any clear indication of what the transition process might look like and how long it would take � or if either of the Syrian sides would be ready to support such a plan. Assad, a member of Syria's Alawite minority, won re-election last year in a vote that Western countries called a sham and his term ends in 2021. The Sunni-led opposition wants him out immediately.

Underscoring the search for a compromise, French Foreign Minister Laurent Fabius only said Assad should step aside "at one moment or another." Saudi Foreign Minister Adel al-Jubeir said earlier this week that Assad must quit his post "within a specific timeframe."

Since the start of Syria's unrest in 2011, Assad's future has been a stumbling block to all efforts aimed at ending the fighting. President Barack Obama demanded that Assad leave power only months into the fighting. Russia resisted the push by blocking attempts at the United Nations to pressure the Syrian leader and insisting that any new government only be established by mutual consent of both the government and the opposition. That essentially gave Assad veto power over his would-be replacements.

Even if the countries do reach an agreement on Assad, it won't solve everything. Syrians of all stripes will have to figure out how to stop the violence between the army and the many different militant groups and how to share power in a government comprised of such fierce enemies.

They need a new constitution. They must figure out what to do about groups such as the Nusra Front, al-Qaida's affiliate in Syria that has at times worked alongside Western-backed fighters. They must decide whether to hold people accountable for crimes committed by all sides. And they have to find a way to cooperate to help defeat the Islamic State.

None of those decisions seem reachable without first determining Assad's future. That includes what "transition" would mean for him, what powers he could maintain as part of that transition, how long the process should take and whether he can compete in a future presidential election, something his international backers refuse to rule out.

Friday's outcome is likely to be more modest.

Russia's deputy foreign minister, Mikhail Bogdanov, said his country and Saudi Arabia had exchanged lists of opposition groups that should be involved in future peace talks � an important question as those fighting Assad range from al-Qaida-linked militants to self-styled moderates.

There was no immediate word on any direct exchanges between Shiite-dominated Iran and Sunni-led Saudi Arabia, bitter regional rivals who have waged proxy battles for influence across the Middle East. As the full meeting of all countries began Friday, Saudi Arabia's al-Jubeir was seated about as far apart from Iranian Foreign Minister Mohammad Javad Zarif as possible.

By AP