The Syrian Observatory for Human Rights said the raid killed at least 83 soldiers
The Pentagon has admitted that US-led coalition aircraft hit a Syrian army position in the east of the country on Saturday.
Syrian state media reported the raid near the Deir Ezzor airport around 1700 hours local time.
"Warplanes from the American coalition hit one of the Syrian army's positions... near the Deir Ezzor airport," the statement said.
A Pentagon statement said, "Coalition forces believed they were striking an Islamic State fighting position."
"The coalition air strike was halted immediately when coalition officials were informed by Russian officials that it was possible the personnel and vehicles targeted were part of the Syrian military," the statement read.
The monitoring group in the area, the Syrian Observatory for Human Rights said at least 83 soldiers were killed in the raids on Jabal Therdeh, southwest of the Deir Ezzor airport.
The coalition strikes are believed to have hit two hilltops near the airport.
A Syrian military source also claimed rounds were fired at the wounded soldiers and paramedics from the planes.
The raids have allowed the Islamic State militant group to advance on government troops in the region.
"This is a dangerous and bold attack against the Syrian state and army, and clear evidence that the United States and its allies support the terrorist group Islamic State," Observatory head Rami Abdel Rahman said.
The militant group on Saturday afternoon claimed it had seized full control of Jabal Therdeh, which overlooks the Deir Ezzor airport.
Syria's biggest ally, Russia, said 62 Syrian soldiers were killed and at least 100 more wounded in strikes by "warplanes from the international coalition".
Russia has cast doubt over the prospects of the ceasefire negotiated with the United States. Under the fragile deal which into force on Monday, the fighting is to be halted on all fronts.
But Moscow raised questions over Washington's commitment to the deal and said the situation had worsened.
Moscow and Washington are to begin joint military operations against Islamic State in Syria, as the next step in the ceasefire deal, provided it holds for a week.
(WION with inputs from agencies)