A US Defense Department spokesperson says US military bases in Syria have been attacked by Iran-backed militias twice since the US carried out retaliatory airstrikes in Iraq and Syria on Friday.
The US airstrikes were in response to a deadly drone attack in Jordan last month that killed three American service members.
Pentagon Press Secretary Pat Ryder told reporters on Monday that two rockets were fired on Saturday targeting US forces at a facility in Syria.
He said another US military facility in Syria was attacked by a drone on Sunday. He added that there were no reported US injuries.
Asked about the reported deaths of Kurdish fighters who worked with US troops, Ryder said that he is aware of reports of Kurds being killed in Sunday's attack, but he declined to elaborate on the matter.