Moscow stepped up its war of words with Washington on Sunday, saying air strikes by a U.S.-led coalition on the Syrian army threatened the implementation of a U.S.-Russian ceasefire plan for Syria and bordered on connivance with Islamic State.
The Russian Defense Ministry said on Saturday that U.S. jets had killed more than 60 Syrian soldiers in four air strikes by two F-16s and two A-10s coming from the direction of Iraq.
The Syrian Observatory for Human Rights, a Britain-based monitoring group with contacts across Syria, cited a military source at Deir al-Zor airport as saying at least 90 Syrian soldiers had been killed.
Russia's Foreign Ministry said in a strongly worded statement that the United States' position on the incident was "unconstructive and inarticulate".
"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 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."
Russia has repeatedly called on the United States to push units of moderate Syrian opposition to separate from Islamic State and other "terrorist groups".
The Foreign Ministry said Saturday's incident was a result of Washington's "stubborn refusal" to cooperate with Moscow in fighting Islamic State, the Nusra Front - now renamed Jabhat Fatah al Sham - and "other terrorist groups".