Three members of the US military were injured on Wednesday in rocket attacks on two sites in Syria where American troops are housed, US Central Command said.
It said rockets landed in its Conoco and Green Village sites in north-east Syria, which led to a US response by attack helicopters.
The exchange of fire between American troops and Iranian-backed fighters in Syria comes a day after the US carried out air strikes on “Iran-linked” targets in the country.
Centcom believes up to three militants were killed in its response to the rocket attacks.
The US military said one service member was treated for a minor injury, while two others were under observation.
“We're not going to hesitate to defend ourselves," Colin Kahl, undersecretary of defence for policy, said on Wednesday before the exchange of fire was reported by Centcom.
"And we've communicated this in both in the actions that we took last night the nature of those actions, and also what we have communicated to the Iranians.
“We're not going to tolerate attacks by Iran-backed forces on our forces anywhere in the world.”
US President Joe Biden authorised military strikes on Tuesday against bases the Defence Department said were being used by Iran-aligned groups to attack the US-led coalition in Syria.
Mr Kahl said the strike in Syria was “proportionate and precise” and was carried out in response to an August 15 attack on a base used by US-backed Syrian opposition fighters.
“The strike was necessary to protect and defend US personnel in Syria, which had been the target of several recent attacks by Iran,” he said.
“This operation is a demonstration that the United States will not hesitate to defend itself against Iranian and Iran-backed aggression when it occurs.”
Mr Kahl said the Tuesday operation struck nine out of the 11 bunker targets affiliated with the Iran-backed fighters.
Iranian Ministry of Foreign Affairs spokesman Nasser Kanaani said the strikes were a “violation of [Syrian] sovereignty, independence and territorial integrity” and called for US troops to leave the country.
