MUNICH — The United States, Russia and other powers have reached agreement on a “cessation of hostilities” in Syria’s civil war that allows for immediate humanitarian access to besieged areas, U.S. Secretary of State John Kerry announced from Munich early Friday morning.

The end of hostilities, which Kerry avoided calling a cease-fire, is scheduled to go into effect “in one week’s time,” Kerry said. Humanitarian access to towns and cities in Syria where food and medical supplies have been blocked, sometimes for months, is to begin immediately.

“It was unanimous,” Kerry said. “Everybody today agreed on the urgency of humanitarian access. What we have here are words on paper. What we need to see in the next few days are actions on the ground.”

Agreement came after daylong consultations in Munich that lasted until early Friday. Hours earlier, Russian Foreign Minister Sergei Lavrov huddled with his counterpart from Iran, Russia’s ally in backing the Syrian government of President Bashar al-Assad, and Kerry sat down with allies backing the Syrian opposition, before all parties gathered for a joint meeting at which the deal was struck.

Lavrov called cessation of hostilities the “first step” toward a full cease-fire.

The effort has been considered a last chance to stop the 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 resolve in 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 negotiations.

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

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

The plan, drafted by United Nations envoy Staffan de Mistura, in consultation with Kerry and the Syrian opposition, assumes that Assad’s government, which is not represented here, would be pressed by Russia to agree. Parties to the talks said that the first relief drops could occur as early as this weekend.

Opposition leaders had said they were optimistic after talks with Kerry and others. “We’ll wait two days and see if all the promises they made are kept,” Salem al-Meslet, 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.”

Meslet said the opposition would return to talks with the government if the plan is implemented. But, he said, “we have to see something — food go to children who are starving to death. Then we’ll go sit at the same table” with the government.

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

Under the draft plan as it stood early Thursday, Russia would take responsibility for humanitarian airdrops, avoiding potential problems of U.S. or allied military aircraft flying over combat zones in sovereign Syrian territory. Although the United States and its allies have conducted thousands of airstrikes over Syria in the past 18 months, all have targeted the Islamic State in areas beyond the government’s control, and with its tacit acceptance.

Under the agreement as initially drafted, two committees would be formed of the 17 countries that are part of the so-called International Syrian Support Group, or ISSG, formed in November at Kerry’s urging. The group, including Russia and Iran in addition to U.S. allies in Europe and the region surrounding Syria, developed a formula for peace talks between the Syrian government and the opposition, with a U.N. resolution mandating its terms for a cease-fire, formation of a transition government and eventual negotiations.

One of the new committees would monitor humanitarian access and deliveries, troubleshooting and adjudicating claims of interference. The second committee would monitor the cease-fire. Details of what some diplomats called a less-formal “cessation of hostilities” have still not been firmed up.

The goal is to ensure that charges of violations would be directed to the committee, rather than responded to in kind. Any fighting group that signed on to and complied with a cease-fire would be exempt from airstrikes. It presumes that the Islamic State and Jabhat al-Nusra, considered by all parties to be terrorist groups, would not participate. Opposition groups embedded with al-Nusra in the anti-Assad fight would have to decide whether to sever those links and separate themselves geographically from the militants.

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 the current level of opposition training and equipping, so as not to leave the rebels at a disadvantage if the cease-fire collapses. Russia would presumably continue its support for the Syrian government.

Despite the diplomatic talks here, both real and verbal combat continued Thursday. Russia’s Defense Ministry was defiant about its 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 any time 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 actually responsible.

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

Washington Post writers Michael Birnbaum, Thomas Gibbons-Neff, Griff Witte and Brian Murphy contributed to this report.