The US has carried out an air strike targeting Iran-backed militias in Syria, in the first military action undertaken by the Biden administration.
The Pentagon said the strike destroyed "multiple facilities" and was ordered in response to attacks against US and coalition personnel in Iraq.
Militia officials said one person had been killed but a war monitor reported at least 22 fatalities.
Syria condemned the attack as a "bad sign" from the new US administration.
The Pentagon said its strike near the Iraqi border in eastern Syria was a "proportionate military response" that was taken "together with diplomatic measures", including consulting coalition partners.
It came after a civilian contractor was killed in a rocket attack on US targets earlier this month. A US service member and five other contractors were also injured when the rockets hit sites in Irbil, including a base used by the US-led coalition.
Rockets have also struck US bases in Baghdad, including the Green Zone, which houses the US embassy and other diplomatic missions.
There are about 2,500 US troops in Iraq to assist Iraqi forces in the fight against the Islamic State (IS) group.
Sending a message
Analysis by Paul Adams, BBC News
The messaging around this strike is probably more important than the strike itself.
Ten days elapsed between the trigger - 15 February's rocket attack in Irbil - and retaliation.
The US defence secretary made a point of thanking the Iraqi government for its intelligence input.
The Pentagon said the air strikes had been conducted "together with diplomatic measures", including consultation with coalition partners.
Nor did the attacks take place on Iraqi soil, thus minimising any embarrassment for the government in Baghdad.
In short, Washington seems to be drawing a sharp distinction with the more intemperate, unilateral instincts of the previous administration.
But at a time when the Biden administration is exploring ways of reviving the 2015 Iran nuclear deal, the strikes also sends a message to Tehran: just because we're willing to sit down and talk doesn't mean your proxies around the region can do what they want.
What do we know about the air strike?
The Pentagon said the strike on Friday was launched "at President Biden's direction".
It targeted facilities located at a border control point used by a number of Iran-backed militia groups, including Kataib Hezbollah and Kataib Sayyid al-Shuhada, who are allied with the Damascus government.
Kataib Hezbollah and Kataib Sayyid al-Shuhada have previously carried out or supported rocket attacks targeting US assets in the country. The US has launched retaliatory strikes.
But Kataib Hezbollah has denied involvement in recent attacks against US interests.
In its statement, the Pentagon said its latest operation "sends an unambiguous message".
"President Biden will act to protect American and Coalition personnel. 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," it said.
The US did not confirm any casualties, but an Iraqi militia official told the Associated Press news agency at least one fighter was killed and a number of others wounded.
The official said the strikes hit an area along the border between the Syrian city of Boukamal and the Iraqi town of Qaim.
The Syrian Observatory for Human Rights, a UK-based monitoring group, said the US attack had killed at least 22 fighters from the Popular Mobilisation Forces, an Iraqi umbrella group of mostly Shia paramilitaries that includes Kataib Hezbollah.
"The strikes destroyed three lorries carrying munitions," the observatory's Rami Abdul Rahman earlier told AFP. "There were many casualties."
Several attacks targeting US personnel have been claimed by little-known groups. But some Iraqi and Western officials say these are a front for established militias, so they can carry out attacks without being held accountable.
US Defence Secretary Lloyd Austin told reporters he was "confident in the target that we went after".
"We know what we hit," he said. "We're confident that that target was being used by the same Shia militants that conducted the strikes" on 15 February.
Syria's foreign ministry said it "strongly condemns the cowardly American aggression".
"It is a bad sign regarding the policies of the new US administration, which should adhere to international" norms, it said in a statement.
Russian Foreign Minister Sergei Lavrov said the US needed to spell out directly its plans for the region, and criticised the "four or five minutes" warning of the strike that the US had given Moscow, Syria's key ally.
Pentagon Press Secretary John Kirby said the strike was meant to punish the militias but not to escalate tensions with Iran, with whom the US is seeking to renew talks over a nuclear deal abandoned by former President Donald Trump, the New York Times reports.
Since 2009, the US has designated Kataib Hezbollah as a terrorist organisation, accusing them of threatening the peace and stability of Iraq.
Iran's influence over Iraq's internal affairs has grown steadily since the US-led invasion that toppled Saddam Hussein in 2003.