WASHINGTON » The United States, Russia and other world powers agreed to implement a nationwide “cessation of hostilities” in Syria’s civil war to start in one week in an effort to stop the carnage and allow delivery of humanitarian aid to besieged civilians.

The cease-fire will not apply to groups designated as terrorists — namely, Islamic State and the al-Qaida offshoot known as the Nusra Front — so that the U.S.-led coalition can continue airstrikes against their positions.

The Russians also would be allowed to continue airstrikes against what they claim are terrorist groups.

Secretary of State John F. Kerry made the announcement after a meeting in Munich that lasted hours beyond schedule, a sign of the torturous negotiations and deep levels of disagreement among the parties involved in the Syria crisis.

“Obviously it’s been difficult,” Kerry said.

Kerry said details of the “cessation of hostilities” had yet to be worked out. That could include ways to monitor and verify the cease-fire.

“This is a pause that is dependent on the process going forward,” Kerry said. But it will have the effect of stopping offensive actions at least temporarily, he said.

“The objective is to achieve a durable long-term cease-fire at some time,” he said.

Russia, which is backing the Syrian government, is proposing a cease-fire to begin March 1. After days of bombing runs that U.S. officials say have killed civilians and moderate U.S.-backed ebels, Moscow has given its ally a clear military advantage.

The suspicion is that Russia would prefer a cease-fire to begin three weeks from now in order to provide time to finish crushing the rebels and return the besieged city of Aleppo, Syria’s largest, to President Bashar Assad’s control.

The United States, Saudi Arabia, other Arab gulf states and much of the West want to get rid of Assad, saying his brutality and willingness to use chemical weapons against his people make him more suited for a war crimes tribunal than a presidential palace.

But Russia and Iran remain Assad’s firm backers, and their forces have shifted the balance of power in war-torn Syria back to Assad after nearly five years of civil war.

U.S. and European diplomats rejected the Russian proposal, saying the cease-fire had to be immediate.

“The future of Syria and Syrians is in our hands,” European Union foreign policy chief Federica Mogherini said in Munich.

Kerry started today’s meeting with his Russian counterpart, Sergei Lavrov, and the two were scheduled to join other members of the so-called International Syria Support Group, a collection of 20 nations working on the conflict, which has fueled the largest refugee crisis in Europe since World War II and killed more than 200,000 people.

The Obama administration also wants a settlement in Syria to allow for more focused fighting to eliminate Islamic State militants, who have used the chaos to take over large swaths of Syria and Iraq before expanding now to Libya.

Kerry tweeted that he “made clear (to Lavrov) the need for immediate progress on humanitarian access, ceasefire.”

“We’re going to have a serious conversation about all aspects about what’s happening in Syria,” he told reporters in Munich. “We will talk about all aspects of the conflict.”

Russian officials have described their cease-fire proposal as “concrete” and “very specific.”

But Russian presidential press secretary Dmitry Peskov was quoted Thursday in Russia media as saying agreement remained elusive.

“The process is very fragile,” Peskov said. “It would be wrong to talk about any unanimity in the settlement process.”

Far from agreement, Russian and U.S. officials traded accusations at the margins of the Munich meeting. Moscow rejected a U.S. military statement Wednesday saying Russian warplanes had destroyed two hospitals in Aleppo. Russia insists the damage was the result of U.S. airstrikes. U.S. military and State Department officials denied the Russian claim.

More than 50,000 Syrian refugees have fled Aleppo and the surrounding area toward the border with Turkey, where they are in effect trapped. Already reeling from the influx of more than 2.5 million Syrian and Iraqi refugees, Turkey has refused to allow more to enter. President Recep Tayyip Erdogan said doing so would further the “ethnic cleansing” that Assad is causing. He instead called for the establishment of havens in Syria.

The top human rights official for the United Nations, Zeid Raad Hussein, said the deterioration of conditions in Aleppo, where about 300,000 people may be trapped and under attack, was “grotesque.”

“The warring parties in Syria are constantly sinking to new depths, without apparently caring in the slightest about the death and destruction they are wreaking across the country,” said Hussein, U.N. high commissioner for human rights.

NATO said Thursday that it plans to deploy warships and surveillance planes to the Aegean Sea to monitor the crossings of thousands of Syrian refugees, who are risking their lives on a daily basis to escape civil war.

The NATO force will work with the European Union’s border management agency, Frontex, to stem illegal trafficking and illegal migration, said NATO Secretary-General Jens Stoltenberg.

“This is not about stopping or pushing back refugee boats,” he said at a NATO ministerial meeting in Brussels. “NATO will contribute critical information and surveillance to help counter human trafficking and criminal networks.”

Another goal of the Munich talks is to persuade the warring parties to return to negotiations in Geneva on Feb. 25. The first attempt at negotiations broke down this month when opposition groups walked out over the Aleppo offensive.

There has been persistent disagreement over even the basics of who should be allowed at the negotiating table; Syria considers many of the opposition groups to be “terrorists,” the same label Turkey gives to several of the Kurdish groups.

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(Times staff writer W.J. Hennigan in Washington contributed to this report.)