Secretary of State John F. Kerry, left, and the U.N. Special Envoy for Syria, Staffan de Mistura, after talks in Geneva on May 2. (Salvatore Di Nolfi/AP)

The United States and Russia are studying possible ways to separate rival forces in Syria, delineating potential “safe zones” for opposition fighters amid renewed violence that has threatened to fully collapse a two-month-old cease-fire attempt.

Secretary of State John F. Kerry — in Geneva on Monday for emergency meetings on the crisis — said that the next 24 to 48 hours will be crucial in determining whether the plan will work.

“I don’t want to make any promises that can’t be kept,” he said.

[U.N. envoy fears “catastrophic” spiral in Aleppo]

Kerry emphasized that the truce initially succeeded and continues in some parts of the country. But violence escalated recently, particularly in Aleppo, where at least 250 civilians have been killed over the past week, including staff and patients at a main hospital, largely by Syrian government airstrikes.

“There are only two air forces flying in that particular area,” Kerry said, referring to the government and Russia, its primary backer. “The Russians have been clear they are not flying.”

Russia and the United States play important, but opposing, roles in Syria. Moscow last year sent in warplanes to help the forces of Syrian President Bashar al-Assad, while Washington and its regional partners, including Saudi Arabia, support some rebel factions seeking Assad’s ouster.

[Gallery: Mourning and misery in Aleppo]

Kerry spoke after meetings with the top U.N. envoy for Syria, Staffan de Mistura, and Saudi Arabian Foreign Minister Adel al-Jubeir. State Department officials said Kerry spoke to his Russian counterpart, Sergei Lavrov, by phone. He planned to return to Washington on Monday evening.

“We are trying in the next hours to see if it is possible to reach an agreement that can­ . . . create a path forward for the cessation to hold so that there isn’t one day of silence or two days of silence, but an ongoing process that relieves the people of Syria from this devastation, from this day-to-day killing machine that is being unleashed by the Assad regime,” he said.

Just as the United States and its allies must “keep our part of the bargain” by ensuring the opposition’s compliance, Kerry said, “it is incumbent on Russia and Iran . . . to make sure that the regime is living up to its part of this agreement.”

De Mistura, who spoke at Kerry’s side after their meeting, said: “We are preparing the mechanism, but the mechanism needs a political will. Otherwise, we will have only a mechanism. But that actually [is] being started today, preparing for a much better mechanism for monitoring and controlling a new cease-fire, but we need political will.”

De Mistura plans to visit Moscow on Tuesday to discuss the new plans.

He has said that nascent political talks between the opposition and the Syrian government to end the more than five-year-old conflict cannot continue unless the violence stops.

Kerry was vague on how the new plan would work or be enforced, saying only that Washington and Moscow had agreed to significantly increase the number of personnel monitoring the cease-fire that took effect Feb. 27.

He said they were working “intensely” to ensure that the task force does “a better job, with a better ability to enforce” the cease-fire.

The safe zones under discussion, however, are not near the Turkish and Jordanian borders, which had been previously proposed by some regional leaders as possible no-fly zones. The Obama administration has consistently rejected using U.S. aircraft to protect such zones.

[Two sides of Aleppo: Destruction and dining out]

Instead, the plan would begin with areas in and around Aleppo and later expand to other regions to seek to divide opposition forces and the Syrian military.

That plan, its proponents hope, would also attempt to isolate militias loyal to Jabhat al-Nusra, al-Qaeda’s Syrian affiliate, which is not covered by the cease-fire. The Syrian government and Russia have used the overlap of Jabhat al-Nusra and rebel forces in some areas to justify the renewed bombing.

The United States and its allies have rejected those claims, noting that much of the bombing has been against civilian areas with minimal rebel presence, as well as places universally recognized as out of bounds, such as hospitals.

Kerry acknowledged that the opposition also stepped up fighting and has been responsible for some of the recent carnage. But he insisted that the United States has upheld its end of the deal by urging U.S.-backed rebel factions to honor the truce.

“We want to make sure that Russia” is doing the same with Assad, he said. “One party is blatantly violating” the agreement.

Jubeir, whose government has supported some suspected Islamist groups and others in Syria, expressed strong doubt that Assad would comply with any plan that would stop airstrikes.

[A grim tour inside Syria’s chaos]

“The regime will not accept safe zones,” Jubeir said. He accused Assad of conducting “ethnic cleansing” in areas currently outside government control, including Aleppo.

The cease-fire was designed to provide space for the opposition and the Syrian government to hold political talks leading to a U.N.-backed transitional government, a new constitution and elections within 18 months.

De Mistura said last week that there has been some progress in the talks, although they have so far been held only in “proximity,” with representatives from the government and the opposition sitting in separate rooms while he and his staff shuttle between them.

Early last week, as the Aleppo attacks intensified, the opposition suspended its participation. There is no point in setting a date for resumption, de Mistura said, until the violence ends.

In his more negative view of a truce, Jubeir said Saudi Arabia continues to favor supplying the rebels with heavier arms, including ground-to-air-missiles — an escalation in weaponry that the United States has long rejected.

“We have said from Day One that the rebels should get all the weapons they need,” he said, specifically noting shoulder-launched missiles.

Asked separately about recent indications of strains in U.S.-Saudi relations, Jubeir described the ties as “excellent, very strong.”

“We may have differences on tactical issues” over Syria and other regional matters, “but our objectives are completely aligned. . . . The American commitment to the security of Saudi Arabia is unshakable.”

Jubeir repeated his government’s call for the release of 28 pages of classified documents from the FBI’s investigation into the Sept. 11, 2001, hijackers, 15 of whom were Saudi citizens.

While some members of Congress have said that the pages may prove official Saudi complicity in the attacks, both the administration and the Saudi government have rejected such claims and pointed to final investigative reports that labeled them without substance.

“So many years have gone by, and everybody knows what’s in the 28 pages,” despite “a lot of innuendo and insinuations,” Jubeir said. “So, yes, release the 28 pages. And it would be nice if, when you release the 28 pages, you release the rebuttal.”