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Syria on Friday condemned a deadly US air strike on Iran-backed militias in its far east as a bad omen from the administration of new US President Joe Biden.

The US said it carried out the strikes overnight at a Syria-Iraq border control point used by Iran-backed groups, destroying "multiple facilities" in retaliation for a spate of rocket attacks targeting its troops in Iraq.

A war monitor said at least 22 fighters were killed in the operation that struck three trucks loaded with munitions coming from Iraq near the Syrian frontier town of Albu Kamal.

It was the first US military action targeting such groups since Biden took office five weeks ago and came just as Washington had opened the door to resuming negotiations with Tehran over its nuclear programme.

"At President Biden's direction", the US raids targeted "infrastructure utilised by Iranian-backed militant groups in eastern Syria", Pentagon spokesman John Kirby said.

"These strikes were authorised in response to recent attacks against American and coalition personnel in Iraq, and to ongoing threats to those personnel."

The air strikes were an "unambiguous message" from Biden, White House press secretary Jen Psaki said.

Biden "is sending an unambiguous message that he's going to act to protect Americans and when threats are posed, he has the right to take an action at the time and (in) the manner of his choosing", she said.

The Syrian Observatory for Human Rights said the strike killed at least 22 fighters from Iraq's state-sponsored Hashed Al-Shaabi paramilitary force.

The raid also destroyed border posts of the Hashed, an umbrella group that includes many small militias with ties to Iran, said the monitor.

Kirby said the location was used by Kataeb Hezbollah and Kataeb Sayyid Al-Shuhada, two Iraqi pro-Iran groups operating under the Hashed.

Kataeb Hezbollah said one of its fighters was killed and slammed the strike in a statement as "a barbaric aggression", calling it "a heinous crime in violation of international law".

'Bad sign'

It identified the dead fighter as Rahi Al-Sharifi and said he had been "stationed at the Iraqi-Syrian border to protect Iraq's land and people from the gangs" of the Islamic State group.

Syria condemned the strike as "cowardly American aggression".

"It is a bad sign regarding the policies of the new US administration which should adhere to international" norms, its foreign ministry said. 

Iraq's defence ministry denied the US had coordinated with it to conduct the strike, saying it only works together with the US-led coalition in the fight against IS.

Syria's ally Russia also condemned the attack, with Foreign Minister Sergei Lavrov questioning the Biden administration's plans in Syria.

"It is very important for us to understand the United States' strategic line on the ground," he said.

The US action followed three rocket attacks on facilities in Iraq used by US and coalition forces fighting IS.

One of those strikes, on a military complex in the Kurdish regional capital Arbil on February 15, killed a civilian and a foreign contractor working with coalition forces, and wounded several US contractors and a soldier.

Last week, the Biden administration offered talks with Iran led by European allies as it seeks to salvage a 2015 nuclear deal, left on the brink of collapse after Biden's predecessor Donald Trump withdrew from it.

But the new administration has also made clear it would not brook "malign activities" in the region by Iran.

'We know what we hit'

Although Kataeb Hezbollah did not claim responsibility for the attacks, US Defence Secretary Lloyd Austin said it was behind them.

"We're confident in the target we went after. We know what we hit," he said.

"We are confident that the target was being used by the same Shia militia that conducted the strikes" against US interests in Iraq, he added.

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 Revolutionary Guards commander, was Iran's key liaison to its allies in Iraq and Syria, 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 spokesman Ned Price said 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 also called Thursday's strike "proportionate".

"At the same time, we have acted in a deliberate manner that aims to de-escalate the overall situation."

Nicholas Heras, of the Institute of the Study of War, said other diplomatic interests were also at play.

"The Biden administration is in the process of seeking out a way to show the Israelis that it is willing to do more against Iran and its proxy groups in the region, especially in Syria," he said. 

Agencies contributed to this report.