Four US personnel suffered minor injuries after air strikes on a base in eastern Syria, US officials said on Thursday.
The Syrian Observation for Human Rights, a war monitor based in Britain, and a tribal source said the attack was carried out by Iran-backed militias.
The US-led coalition said the service members were being evaluated for minor injuries and possible traumatic brain injuries after two rounds of "indirect fire" hit support buildings at the Green Village in eastern Syria.
An investigation was under way, the coalition said.
US forces moved into Syria during the campaign against ISIS, linking up with a Kurdish-led group called the Syrian Democratic Forces.
Iran-backed militias have established a foothold in Syria while fighting in support of President Bashar Al Assad during the country's 11-year war.
A tribal source in the area said rockets were fired by an Iran-backed militia, and two landed in the area of al Omar oilfield where US forces are based in Deir Ezzor province, near the Iraqi border.
The rockets were launched from an area west of the Euphrates River, where Iran-backed militias have a presence, the source said.
The Observatory said rockets had been fired after midnight at the base from an area where Iran-backed militias hold influence. It said the US-led troops returned fire.
The Observatory also reported that explosions were heard in the area before noon on Thursday.
US troops came under rocket fire in the same area last year, in apparent retaliation for US air strikes against Iran-aligned militia in Syria and Iraq.
Iranian-backed militias are heavily concentrated west of the Euphrates in Deir Ezzor province, where they receive supplies from Iraq through the Bukamal border crossing.