He said US attacks on Daesh in Mosul mean "they are against ISIS [Daesh], but at the same time, they are supporting ISIS and they allow them to cross the border from Iraq into Syria."
"It means they helped ISIS to come to Deir Ez-Zor and to undermine the Syrian Army who are defending Deir ez-Zor. They wanted Deir ez-Zor to fall into the hands of ISIS," Assad said.
Deir ez-Zor and its military air base have been under siege by Daesh for over three years, with food and ammunition only delivered by air.
On Friday, Assad's adviser said that the Syrian army continues military operations to free Deir ez-Zor and Raqqa from terrorists.
In March, the Syrian army repelled Daesh's attacks near the Deir ez-Zor airfield.