The United States, Russia, Iran and more than a dozen other nations agreed Friday 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 either 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 in Austria's capital 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 since beginning in 2011.

Although details were vague, the approach has clear differences with previous such efforts. Chief among them: The U.S. 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.

"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 U.N. special envoy to Syria, Staffan de Mistura.

The new diplomatic push coincided with a U.S. announcement that a small number of American special operations forces will be sent to northern Syria to work with local ground forces in the fight against Islamic State militants. It would mark the first time American troops would be deployed openly on the ground in the country.

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, U.N.-led process reflects a realization that stopping the bloodshed ought to be the top priority, even if that means relegating its longstanding demand for Assad to step aside so that a peaceful, secular and more inclusive Syria can be established. It may be seen as a concession by rebel militia groups determined to defeat Assad on the battlefield as well as by American critics of President Barack Obama who believe he hasn't acted forcefully enough against Assad or his international backers.

Kerry said the foreign ministers present in Austria's capital all vowed to maintain Syria's institutions, to protect the rights of all its citizens, to assure humanitarian access and to strive to defeat the Islamic State. He said the process should lead to a new constitution for Syria and internationally supervised elections, as well as an end to violence between Assad's military and rebels so that the world community can focus on the counterterrorism challenge.

But no agreement was reached on Assad, whose future lies at the center of the conflict.

Obama demanded that Assad leave power only months into the fighting, but the U.S. has done nothing in terms of direct military action and little in support of the rebels to advance that goal. Russia resisted the push by blocking attempts at the United Nations to pressure the Syrian leader and insisting that any new government be established only by mutual consent of both the government and the opposition. That essentially gave Assad veto power over his would-be replacements.

Underscoring the continued violence, Syrian opposition reported that a government missile barrage killed more than 40 in a Damascus suburb as diplomats were meeting in Vienna.

In the Austrian capital, officials described tense talks, particularly as bitter regional rivals Iran and Saudi Arabia took turns lambasting the Syria policies of the other. Iran has actively fought on the ground alongside Assad's forces and the Iranian proxy militia, Hezbollah. Saudi Arabia has been among the main providers of military assistance to the rebels.

Since last month, Russia, too, has been engaged in the fight. It says it is battling terrorist groups, but the U.S., NATO and others say most of its airstrikes have hit moderate, Western-backed militia.

Kerry and Russian Foreign Minister Lavrov both acknowledged their continued differences, particularly on Assad, while playing up their ability to join together for the ultimate good of Syria.

No one mentioned concrete timetables -- even for when Syrian government and opposition representatives might be pulled into the process.

Officials said the 17 governments in attendance had been considering a plan that would establish a cease-fire within four to six months, followed by the formation of a transition government featuring both Assad and opposition members. Conscious of the deep divide over Assad's fate, they left undefined how long Assad could remain in power under that transition. The officials weren't authorized to speak publicly on the matter and demanded anonymity.

Kerry and Lavrov said another round of Syria talks would occur within two weeks.