U.S. Secretary of State John Kerry has threatened to cut off all cooperation with Russia on Syria unless Moscow and its Syrian allies end an ongoing air assault on the city of Aleppo.
The State Department said Kerry issued the ultimatum to his Russian counterpart, Sergei Lavrov, in a telephone call on September 28.
Kerry's spokesman, John Kirby, said Washington was preparing to "suspend U.S.-Russia bilateral engagement on Syria...unless Russia takes immediate steps to end the assault on Aleppo."
Russia's Foreign Ministry, meanwhile, released a statement saying that Lavrov told Kerry that Washington must fulfill its obligations to separate opposition groups from terrorist groups.
Separating groups from one another on the ground has been hugely problematic, given that battlefield loyalities among the different fighting groups are fluid.
Most, if not all, of the groups share a common goal of ousting Syrian President Bashar al-Assad. And while Washington and Moscow share some agreement in labeling those with connections to Al-Qaeda as terrorist organizations, Assad's government has labeled all of them as terrorists.
Add to that the fact that Syria's government is a longtime ally of Russia, which means that Мoscow's targeting decisions are usually informed by Syria's own tactical goals.
Russia and the United States are backing different sides in Syria's civil war but have tried to find some cooperation in targeting known terrorist groups.
A cease-fire that was hammered out by Washington and Moscow collapsed earlier this month. It was followed by a blistering air assault by Russia and Syrian jets on Aleppo, where hundreds of thousands of civilians are trapped.