A pro-Bashar Assad group that includes Syrian militias along with Russian and Iranian forces has reportedly vowed to strike back at the U.S. if it crosses more "red lines" with further "aggression" against Syria, according to media reports.

"What America waged in an aggression on Syria is a crossing of red lines. From now on we will respond with force to any aggressor or any breach of red lines from whoever it is and America knows our ability to respond well," according to a statement by "the joint command operation center of Syrian allies" and reported by ABC News and Reuters.

"Rest assured that we will liberate Syria from all kinds of occupying forces, it does not matter from where they came to the occupied part of Syria," the statement warned. "Russia and Iran will not allow the United States to be the only superpower in world."

Russian President Vladimir Putin and Iran's Hassan Rouhani accused the U.S. of violating international law during a phone call Sunday, Reuters reported.

Secretary of State Rex Tillerson said Sunday he has no reason to believe Russia would retaliate against the U.S. for a missile strike on a Syrian airfield.

"I see no reason that there would be retaliation since the Russians were never targeted in this particular strike. It was a very deliberate, very proportional, and very targeted strike undertaken in response to the chemical weapons attack. And Russia was never part of the targeting," Tillerson said in an interview with CBS.

The U.S. launched 59 Tomahawk missiles at a Syrian air base on Thursday in response to what the U.S. has concluded was Assad's use of chemical weapons on civilians.

The Trump administration said the missiles targeted the air field to sway against further attacks on Syrian citizens.