U.S. Secretary of State John F. Kerry says that major powers have agreed to accelerate and expand delivery of humanitarian aid in Syria “beginning immediately." (Reuters)

The United States, Russia and other powers agreed to a “cessation of hostilities” in Syria’s civil war, to take place within the next week, and immediate humanitarian access to besieged areas, Secretary of State John F. Kerry announced here early Friday.

“It was unanimous,” Kerry said of a communique issued after hours of meetings among participants in a group of nations that have supported and armed one side or the other in the four-year war. “Everybody today agreed,” he said. But the proof of commitment will come only with implementation. “What we have here are words on paper,” Kerry said. “What we need to see in the next few days are actions on the ground.”

Russian Foreign Minister Sergei Lavrov said the projected date for ending at least some of his country’s airstrikes in Syria is a week from Friday, but he emphasized that “terrorist” groups would continue to be targeted, including the Islamic State and Jabhat al-Nusra, an al-Qaeda affiliate in Syria that is involved in the fight against President Bashar al-Assad. The group in some instances fights alongside rebel forces supported by the United States and its allies.

The determination of eligible targets and geographic areas is to be left up to a task force of nations, headed by Russia and the United States, that will adjudicate differences of opinion. It is expected but by no means guaranteed that signatories to the agreement will be able to persuade their proxies and allies on the ground, including Assad and the hundreds of opposition groups fighting against him, to honor the terms.

The United States, Russia and other powers came to an agreement on a ‘cessation of hostilities’ in Syria, but the deal was met with caution and skepticism. (Jason Aldag,Ishaan Tharoor/The Washington Post)

Kerry and Lavrov emphasized that the agreement is not perfect and will require the goodwill and determination of all involved.

Lavrov also described a “qualitative” change in U.S. military policy to cooperate with Russia in continuing the fight against the Islamic State. Until now, the Obama administration has declined to deal with Russia except to “deconflict” their airstrikes to prevent their aircraft from running into each other in Syria’s skies.

“The key thing is to build direct contacts, not only on procedures to avoid incidents but also cooperation between our militaries,” Lavrov said.

Kerry said there had been no change in policy, but he said humanitarian and other agreed-upon programs would require the ability “to talk about deployment of forces, the presence of people, who can go where, how they get there, and avoid conflict in ways that are effective” to implement the agreement.

Lavrov described the cessation of hostilities as the “first step” toward a cease-fire, a more formal legal construct that can involve the turning in of arms and demobilization of forces. Instead, he described the immediate goal as more akin to a truce.

The aim is that humanitarian relief begin as early as this weekend, with Russian airdrops to at least seven areas of Syria that cannot be easily reached by road. A second task force of countries, drawn from among the 17 that participated in the talks, will determine the “modalities” of allowing ground convoys of food and medicines to pass through government and opposition lines to reach dozens of other besieged communities.

Kerry and Lavrov acknowledged that they and other members of the group continue to disagree about many issues in Syria, including Assad’s future.

Russia's foreign minister Sergei Lavrov said halting hostilities in Syria would be a difficult task and urged for peace talks to resume after an international meeting on the Syria crisis on Feb. 12. (Reuters)

The Munich effort was seen as a last chance to stop carnage in Syria that has left hundreds of thousands dead and sent millions fleeing from the country. What was already a desperate situation in Syria has greatly worsened over the past few weeks, as massive Russian bombardment in and around the city of Aleppo has scattered opposition fighters and driven tens of thousands of civilians toward the barricaded Turkish border.

Participants said they had noted a new U.S. willingness to stand up to the Russians, who agreed in December to a U.N. resolution calling for a cease-fire in conjunction with peace talks.

The Obama administration has been under pressure from its allies to stop the flow into Europe of what are now about 1 million refugees. Partners in the Middle East have also openly despaired of what they see as declining U.S. leadership in the region.

Beyond its recent appearance of allowing Russia to act with impunity, the administration has long resisted calls from regional partners to increase its relatively low level of military aid and training to opposition forces, even as President Obama insisted that Assad would have to step down. A failure of the Munich effort would have presented the administration with a decision on whether to reverse course and expand its assistance to the opposition.

Some diplomats here noted that the Russians may be more amenable now to an early cease-fire, since the airstrikes and Iranian-aided ground operations have achieved their goal of regaining control for Assad over much of the country’s western population centers. This month’s Russian bombing has driven opposition forces out of areas of Aleppo and the surrounding province that they had occupied almost since the civil war began in earnest four years ago.

“Everybody’s calculations have shifted” because of events of the past few weeks, one diplomat said. The diplomats spoke on the condition of anonymity to discuss closed-door talks.

Opposition leaders said as the talks progressed Thursday that they were optimistic after meeting with Kerry and others. “We’ll wait two days and see if all the promises they made are kept,” Salem al-Meslet, the spokesman for a negotiating team appointed by the Syrian opposition to open U.N.-sponsored talks with the government, said before the agreement was announced. “Hopefully, we’ll see something by Monday.”

U.N. special envoy Staffan de Mistura said he anticipated an early resumption of talks between the Syrian government and the opposition. Meslet said the opposition would return to talks if the new plan is implemented. But, he said, “we have to see something — food [must] go to children who are starving to death. Then we’ll go sit at the same table” with the government.

“I can’t stop [Vladimir] Putin,” he said of the Russian president. “Can you say no to Putin?” he said, referring to the United States and its allies.

The initial session of the negotiations was suspended last week after the opposition protested the lack of humanitarian access to besieged areas as well as Russia’s stepped-up airstrikes near Aleppo.

The Munich meeting, the fourth the group of nations has held, was initially intended to bless and monitor peace talks that were supposed to start early this month. Instead, it turned into an emergency session to put the process back on track.

Although isolated, small-scale fighting is likely to continue, the deal would ideally stop the use of heavy weapons, including tanks and antitank missiles. The United States and its partners would continue their current level of equipping and training the opposition so as not to leave the rebels at a disadvantage if the cessation of hostilities collapses. Russia presumably would continue its support for the Syrian government.

Despite the diplomatic talks here, combat both real and verbal continued Thursday. Russia’s Defense Ministry was defiant about Moscow’s intervention in Syria, saying it would not yield to Western entreaties to stop an effort that has given Assad powerful momentum on the battlefield.

Western efforts at “political transitions” led to bloodshed and refugees, Maj. Gen. Igor Konashenkov, a Defense Ministry spokesman, told reporters in Moscow. He gave no indication that Russia plans to stop its combat air missions anytime soon.

Konashenkov denied that Russia was bombing civilians, saying that “no matter how long one baits terrorists, they will not become opposition members.”

Responding to a charge Wednesday from Col. Steve Warren, the Baghdad-based spokesman for coalition operations in Iraq and Syria, that Russian planes had bombed two hospitals in Aleppo, Konashenkov said two U.S. planes were in fact responsible.

“There were no coalition airstrikes in or near Aleppo on Wednesday, Feb. 10,” Warren countered Thursday in a statement. “Any claim that the coalition had aircraft in the area is a fabrication.”

Michael Birnbaum in Moscow, Thomas Gibbons-Neff and Griff Witte in Brussels and Brian Murphy in Washington contributed to this report.