Syrian President Bashar al-Assad said that U.S. coalition air strikes which hit Syrian army positions near Deir al-Zor on Saturday were a "flagrant aggression".
He said the incident which involved the U.S.-led coalition fighting against Islamic State showed an "increase in support for terrorists by countries opposing Syria," state media quoted him as saying.
The United States expressed its "regret" over the incident, which Russia said killed scores of Syrian troops.
It has raised tension between Washington and Damascus's chief ally Moscow, who brokered a ceasefire which after a week appears to be in tatters.
Assad said countries opposing his government, which include the United States, European and Sunni regional powers, were doing "all in their power" to keep the five-year civil war going.
He made the remarks during a meeting with Iranian foreign ministry official Hossein Jaber Ansari in Damascus. Tehran also supports Assad.