A behind-the-scenes dispute between the United States and Russia broke into the open on June 3, with Moscow claiming Washington asked it not to target Al-Qaeda's branch in Syria with air strikes.
"They are telling us not to hit it [Nusra Front], because there is 'normal' opposition next...to it," Russian Foreign Minister Sergei Lavrov said in televised remarks after speaking with U.S. Secretary of State John Kerry by phone.
He said that the U.S. demand runs counter to an earlier agreement that the United States and its allies would press opposition groups which have associated with Nusra in the past to distance themselves from the group and not physically locate near Nusra positions, so the blacklisted group can continue to be targeted with air strikes.
"The opposition must leave terrorists' positions, we long have agreed on that," Lavrov said. "Terrorism is a common threat, and most of our Western partners admit in conversations that there should be no excuses to delay the defeat of Islamic State and Jabhat Al-Nusra, which is trying to merge with other groups."
Nusra and the Islamic State group (IS) were both blacklisted by the United Nations as terrorist organizations, and were specifically excluded from the Syrian cease-fire and peace process, allowing both the United States and Russia to continue hitting them with air strikes.
When confronted with Lavrov's accusation, the U.S. State Department insisted that it only asked Russia to carefully select its targets to avoid hitting civilians and opposition groups that have joined the peace process.
U.S. State Department spokesman Mark Toner said that Kerry in an hour-long conversation with Lavrov on June 3 emphasized the need for Russia to carefully distinguish between the IS, Nusra, and less radical opposition groups.
"This is a common refrain, common theme that we've been conveying to the Russians over the past weeks," Toner said. "We obviously all agree that ISIL [Islamic State] and the Nusra Front pose a real threat to security."
"Of course we support strikes focused solely on either Daesh [Islamic State] or Al-Nusra," he said. "But a greater effort, a more complete effort needs to be made in order to distinguish between Al-Nusra and the parties to the cessation."
Toner noted that strikes against opposition forces and civilians only make people "more supportive of these terrorist groups and that is a dynamic we've seen play out in Syria for years now because of the regime's actions."
Kerry told reporters in Paris that he discussed the upsurge of violence in Syria with Lavrov, and they discussed "ways to try to strengthen the enforcement and accountability" for the cease-fire. But he did not mention the question of air strikes against Nusra.
The matter is of high importance to Moscow, however, as its air strikes have been aimed at furthering the Syrian regime's attempts to oust Nusra militants from Syria's largest city, Aleppo, where they are entrenched along with other rebel groups.
Russia at one point set a deadline for Syrian opposition units to withdraw from areas occupied by Nusra, but then agreed to give them more time to pull out.
With reporting by AP, Interfax, and TASS