With the Syrian ceasefire fraying amid airstrikes in Aleppo, Russian Minister Sergei Lavrov is scheduled Tuesday to meet United Nations envoy Staffan de Mistura in Moscow, Agence France-Presse reported.

The talks come at another tense moment; the United States and its partners have accused Russia of not fulfilling its part in the peace process in Syria by failing to rein in Syrian President Bashar Assad. The U.S. also has accused Assad, a longtime Russian ally, of breaking the terms of the ceasefire announced by Moscow and Washington in February. The Kremlin argued over the weekend that Assad’s airstrikes were targeting militant groups but has since softened its tone ahead of Tuesday's talks.

U.S. Secretary of State John Kerry met with Saudi Foreign Minister Adel al-Jubeir and de Mistura on Monday to discuss how to reduce violence in and around Aleppo.

“There is only one side that is flying airplanes, and that is Bashar al-Assad and his allies, so they are responsible for the massacre of women, children and the elderly,” al-Jubeir said. “They are responsible for the murder of doctors and medical personnel, and this situation, any way you slice it, will not stand. The world is not going to allow them to get away with this.”

The bombing of an Aleppo pediatric hostipal last week left at least 50 people dead, with opposition groups blaming the Syrian government for the attack.

The Syrian government said it had extended its ceasefire around Damascus but did not include Aleppo. Approximately 250 people have died in Aleppo from airstrikes and shelling in the past nine days, the Syrian Observatory for Human Rights reported.

The U.S. said it was deliberating instituting safe zones in Aleppo, but it was unclear whether the Kremlin would be willing to agree to the plan and pressure Assad. Kerry said talks were “getting closer to a place of understanding” with Russia but that work remained.

The five-year civil war in Syria has left over 270,000 people dead and displaced millions as refugees head to neighboring states and Europe to seek asylum. Russia launched airstrikes in Syria at the end of September 2015 vowing to target terror groups such as the Islamic State group but has been widely accused of strategic bombing in support of Assad.