- The US struck eastern Syria, it claimed were being used by Iran-backed armed groups.
- The Syrian Observatory for Human Rights said 17 people were killed in the strike.
- The strike is intended as a message for Iran, said spokesperson John Kirby.
The US military struck facilities in eastern Syria used by Iran-backed armed groups on Thursday, saying President Joe Biden's new administration was sending Tehran a message after recent rocket attacks on US troop locations in Iraq.
In its first military action against Iran-linked groups since Biden became president five weeks ago, the US Defence Department said it had carried out airstrikes at a Syria-Iraq border control point used by those groups, destroying "multiple facilities".
"At President Biden's direction, US military forces earlier this evening conducted airstrikes against infrastructure utilised by Iranian-backed militant groups in eastern Syria," said spokesperson John Kirby in a statement.
"These strikes were authorised in response to recent attacks against American and Coalition personnel in Iraq, and to ongoing threats to those personnel," he said.
Kirby did not say whether there were any casualties in Thursday's attack.
But the Syrian Observatory for Human Rights said that 17 people were killed after the strike hit three trucks loaded with munitions coming from Iraq near the Syrian city of Bukamal.
The group said all the dead were from Iraq's state-sponsored Hashed al-Shaabi force, the umbrella group over many small militias that have ties to Iran.
Kirby said the location was used by Kataeb Hezbollah and Kataeb Sayyid al-Shuhada, two armed Iraqi Shi'ite groups under Hashed al-Shaabi.
The US action followed three rocket attacks on facilities in Iraq used by US and coalition forces fighting the Islamic State group.
One of those strikes, on a military complex in the Kurdish region's capital Arbil on 15 February, killed a civilian and a foreign contractor working with coalition forces, and injured several US contractors and a soldier.
The attacks in Iraq laid down a challenge to the new Biden administration just as it opened a door to resumed negotiations with Tehran over its alleged nuclear weapons programme.
Last week, the administration offered talks with Iran led by European allies as it sought to salvage the 2015 nuclear deal, left on the brink of collapse after the previous government of President Donald Trump withdrew from it.
But the administration has also made clear it would not brook "malign activities" in the region by Iran.
Iran is believed to be searching for an opportunity to avenge the US assassination of top general Qasem Soleimani one year ago.
Soleimani, a senior officer in the Iranian Revolutionary Guard Corps, was Tehran's key liaison to allied groups and figures in Iraq, Syria, Lebanon and elsewhere in the region.
He was killed in a US drone strike just as he arrived in Baghdad for meetings with top Iraqi officials.
State Department spokesperson Ned Price said on Monday the US would "hold Iran responsible for the actions of its proxies that attack Americans" but would not "lash out" and risk destabilising Iraq.
Kirby called Thursday's strikes "proportionate" and said it "was conducted together with diplomatic measures", including consultation with US partners in the anti-IS coalition.
"The operation sends an unambiguous message: President Biden will act to protect American and Coalition personnel," he said.
"At the same time, we have acted in a deliberate manner that aims to de-escalate the overall situation in both eastern Syria and Iraq," he added.