Speaking in Geneva, Gatilov said the strikes violated Syria's sovereignty and would not help efforts to achieve a political solution to the ongoing crisis in the country.
The coalition said the forces "posed a threat" to U.S. and partner forces at Tanf base near the joint Syria-Iraq-Jordan border, where USA special forces were operating and training Free Syrian Army rebels.
The attack comes at a time when the USA presence in Syria is becoming more visible, mostly in the northern parts of the country where American troops are backing Kurdish-led forces fighting the Islamic State group.
"This brazen attack by the so-called worldwide coalition exposes the falseness of its claims about fighting terrorism", the source said.
USA airstrikes targeting pro-Assad forces close to the Iraqi border have been branded "completely unacceptable" by Russian Federation.
"Such actions that were carried out against the Syrian armed forces. this is completely unacceptable, this is a breach of Syrian sovereignty", deputy foreign minister Gennady Gatilov was quoted as saying by state-run RIA Novosti in Geneva.
The source said the attack hampered efforts by the Syrian army and its allies to fight Daesh terrorists who have been wreaking havoc in Iraq and Syria for years.
The coalition said the strike came after unsuccessful "Russian attempts to dissuade Syrian pro-regime movement" as well as "a coalition aircraft show of force, and the firing of warning shots".
The official did not give a number for how many soldiers were killed, but said the Syrian army was fighting terrorism and no one else had the right to decide in which areas it can carry out operations.
The Syrian Observatory for Human Rights, a monitoring group with activists on the ground, said the strike destroyed vehicles and killed eight militiamen.
In September a year ago, United States airstrikes on Syrian army positions near the Dayr al-Zawr military airport also killed over 80 soldiers.
"Of course, all this leads us away from the Syrian settlement and from the main task within this settlement - to prevent the seizure of Syria and other parts of the Middle East by terrorists", he told reporters, adding that it actually meant connivance to terrorists.