Syrian volunteers take part in paramilitary training conducted by the Syrian army north of the capital of Damascus on Monday. (Louai Beshara/AFP/Getty Images)

The United States and Russia announced Monday that they have agreed to the terms of a partial cease-fire in Syria, a deal that will depend on their ability to cooperate amid deep mutual suspicion and test their willingness and ability to dictate terms to their allies on the ground.

Under the agreement, Washington and Moscow are to establish a hotline between them to monitor compliance and resolve potential problems, a joint statement said. The two are also “prepared to work together to exchange pertinent information” delineating territory currently held by various Syrian combatants and ensuring that neither country, nor any of their allies, bombs groups or areas covered by the accord.

Opposition groups, and the government of Syrian President Bashar al-Assad, have until noon Friday to inform the United States or Russia that they agree to the terms, or risk coming under renewed attack. The agreement excludes the Islamic State, al-
Qaeda’s affiliate Jabhat al-Nusra, and any other “terrorist” group that in the future may be designated by the United Nations.

The opposition High Negotiations Committee, after an emergency meeting Monday in Riyadh, said it accepted the agreement. Syria’s official news agency, SANA, reported the U.S. and Russian announcements, with no government response.

President Obama spoke by telephone with Russian President Vladi­mir Putin earlier Monday, “at Putin’s request,” about the “understanding” that had been reached, White House press secretary Josh Earnest said.

Earnest offered tempered optimism about an agreement that he said provided a “moment of opportunity.” Its success, he said, depended on “all parties” following through “on the commitments they have made.”

While the United States is depending on Russia to bring the Syrian government and Iran to heel, rebel backers including Turkey and Saudi Arabia are under pressure to rein in groups thought to be under their control. The deal does not address the resupply of combatants by their outside backers.

Putin moved quickly to paint the agreement as a watershed moment in the history of the five-year Syrian conflict, saying in a triumphant statement to Russian television networks that it could finally put an end to the violence.

“I am convinced that the joint actions agreed upon with the United States can radically turn around the critical situation in Syria,” he said.

The U.S.-Russia hotline and establishment of a joint task force with military components are likely to be seen by Putin as something of a victory. The United States and NATO cut all military coordination with Russia after its intervention in Ukraine.

Since September, when Russian airstrikes in support of Assad began, “deconfliction” talks have been held to ensure that U.S. and Russian aircraft operating over Syria do not engage. But the United States, until now, has refused any coordination or sharing of other information in Syria and has charged that Russian airstrikes have targeted primarily non-terrorist groups opposed to Assad’s rule.

Both Putin and Assad have labeled all rebel groups in Syria as terrorists, and Putin’s statement couched the new agreement as joint cooperation in the counterterrorism fight. The deal “can be an example of a responsible response to the threat of terrorism, based on international law and U.N. principles of action of the international community,” he said. “We hope that the Syrian leadership and all our partners in the region and beyond will support the course of action of the chosen representatives of Russia and the United States.”

U.S.-Russian agreement in principle on a “cessation of hostilities” was reached Feb. 12 at a meeting in Munich also attended by 15 other outside stakeholders in the Syrian conflict. Since then, as heavy Russian bombardment has continued, Secretary of State John F. Kerry and his Russian counterpart, Sergei Lavrov, have engaged in extensive telephone diplomacy while teams from both governments — including senior military officials — met in Geneva to hammer out details.

Critics of the deal — and the length of time it has taken to reach it — have charged that Russia dragged out the negotiations in order to expand territory held by the Syrian government with air bombardment and ground forces composed of Iranian-led Shiite militias from Iraq and Lebanon.

The agreement calls on all parties on the ground to “refrain from acquiring or seeking to acquire territory from other parties to the ceasefire.”

While the Islamic State is edging from its eastern Syria strongholds into western areas where the civil war is raging, its forces are still largely separate, and the cease-fire is likely to have little impact on U.S. air operations against the militants.

In southern Syria, where both Jabhat al-Nusra and opposition groups backed by the United States and its partners are largely fighting separately against government forces, Russian strikes against Jabhat al-Nusra are likely to continue.

Problems will doubtless occur in the northwest, where fighting is currently most fierce and where Jabhat al-Nusra fighters are intermingled with the opposition rebels supported in varying degrees by the United States and its European and regional allies.

“Russia and the regime will target the areas of the revolutionaries on the pretext of the Nusra Front’s presence . . . and if this happens, the truce will collapse,” Bashar al-Zoubi, head of the political office of the Yarmouk Army, part of the rebel force, told the Turkish newspaper Today’s Zaman.

The United States and Russia hope to deal with the overlap by exchanging what the agreement calls “aggregated data that delineates territory where groups that have indicated their commitment to and acceptance of the cessation of hostilities are active, and a focal point for each side, in order to ensure effective communication.”

“We are all aware of the significant challenges ahead,” Kerry said in a statement.” Over the coming days, we will be working to secure commitments from key parties” that they will abide by the terms of the agreement.

In addition to the cease-fire, parties to the deal must also agree to participate in U.N.-facilitated political negotiations to forge an end to the civil war. An initial round of talks foundered early this month, when opposition negotiators refused to participate until Russian bombing stopped and humanitarian access was allowed to cities and towns besieged, in most cases, by government forces.

Initial convoys of aid reached some of those areas last week. The cease-fire calls for all parties to ensure “rapid, safe, unhindered and sustained access” through territories they control.

Opposition forces must “cease attacks with any weapons, including rockets, mortars, and anti-tank guided missiles, against Armed Forces of the Syrian Arab Republic, and any associated forces.”

The Syrian government “and all forces supporting or associated” with it must “cease attacks with any weapons, including aerial bombardments by the Air Force of the Syrian Arab Republic and the Aerospace Forces of the Russian Federation, against the armed opposition groups” participating in the deal.

Rather than responding in kind to perceived violations, all parties are instructed to funnel their charges “to the attention of the Task Force.” The cessation of hostilities, the agreement said without details, “will be monitored in an impartial and transparent manner and with broad media coverage.”

Michael Birnbaum in Moscow and Brian Murphy in Washington contributed to this report.