"Moscow is deeply concerned about what happened," said a foreign ministry statement.
"We call on the American partners to conduct the most thorough investigation and take measures to exclude such incidents in the future."
"The actions of the pilots -- if they, as we hope, were not taken on orders from Washington -- fall between criminal negligence and direct pandering to IS terrorists," it said.
The US admitted the coalition strike on regime positions near Deir Ezzor expressing "regret" over the loss of life that it said was a mistake.
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The ceasefire came into effect on Monday, and if it lasts seven days, Russia and the United States are to work together to target the extremist Islamic State and Al-Nusra front.
"They could wait two more days" and consult with Russia, to "make sure they are striking the right people," UN Ambassador Vitaly Churkin told journalists late yesterday, after walking out of the Security Council in protest over remarks made by US delegate Samantha Power.
Russia's foreign ministry again called on Washington to force Syrian rebels to join the truce.
"Otherwise, the realisation of Russia-US agreements... could be put in danger.
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