VIENNA -- The United States, Russia, Iran and more than a dozen other nations agreed yesterday to launch a new peace effort involving Syria's government and opposition groups, but carefully avoided any determination on when President Bashar Assad might leave power -- perhaps the most intractable dispute of the conflict.

There was no guarantee that Assad or the vast array of rebel groups fighting against him would join the push for peace.

The plan was hashed out after two days of discussions among some of the fiercest geopolitical foes on the planet, including governments fighting directly or by proxy on opposing sides in a civil war that has killed more than 250,000 people, uprooted 11 million from their homes, led to the emergence of the Islamic State and sparked a refugee crisis throughout Europe. The Syrian opposition reported yesterday that a government missile barrage killed more than 40 people in a Damascus suburb.

Details from the talks were vague, but the approach has differences from previous such efforts. Chief among them: The United States and allies including Saudi Arabia softened calls for Assad's quick removal from power. Russia and Iran didn't rule out his eventual departure.

"Four-and-a-half years of war, we all believe, has been far too long," U.S. Secretary of State John Kerry told reporters.

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"I did not say that Assad has to go or that Assad has to stay," Russian Foreign Minister Sergey Lavrov said at the news conference with Kerry and the UN special envoy to Syria, Staffan de Mistura.

Kerry and Lavrov acknowledged their continued differences on Assad while playing up their ability to join together for the ultimate good of Syria.

Kerry said the U.S. was intensifying a "two-pronged" effort. Diplomatically, it wants to see peace between the government and rebels as quickly as possible. Militarily, it is determined to defeat the Islamic State.

For Washington, the new, UN-led process reflects a realization that stopping the bloodshed ought to be the top priority, even if that means relegating its long-standing demand for Assad to step aside so a peaceful, secular and more inclusive Syria can be established.