Russia on Sunday warned the U.S. that coalition airstrikes on Syrian army forces are threatening the stability of a ceasefire agreed to by the two nations. In a statement, Russia’s foreign ministry even accused the U.S. of collusion with ISIS. “The actions of coalition pilots—if they, as we hope, were not taken on an order from Washington—are on the boundary between criminal negligence and connivance with Islamic State terrorists,” the foreign ministry said. “We strongly urge Washington to exert the needed pressure on the illegal armed groups under its patronage to implement the ceasefire plan unconditionally. Otherwise the implementation of the entire package of the U.S.-Russian accords reached in Geneva on Sept. 9 may be jeopardized.” The escalated war of words comes as the U.S. acknowledged it was behind a coalition airstrike that killed more than 60 Syrian soldiers.