The United States launched airstrikes in Syria on Thursday, the first military action undertaken by the Biden administration.
The attack was targeting facilities near the Iraqi border used by Iranian-backed militia groups.
The Pentagon said the strikes were retaliation for a rocket attack in Iraq earlier this month that killed one civilian contractor and wounded a US service member and other coalition troops.
“I’m confident in the target that we went after, we know what we hit,” US Defense Secretary Lloyd Austin told reporters flying with him from California to Washington.
Speaking shortly after the airstrikes, he added: “We’re confident that that target was being used by the same Shia militants that conducted the strikes," referring to the February 15 rocket attack in northern Iraq.
Austin said he recommended the action to Biden.
“We said a number of times that we will respond on our timeline,” Austin said. "We wanted to be sure of the connectivity and we wanted to be sure that we had the right targets.”
Earlier, Pentagon spokesman John Kirby said the US action was a “proportionate military response” taken together with diplomatic measures, including consultation with coalition partners.
“The operation sends an unambiguous message: President Biden will act to protect American and coalition personnel," Kirby said. "At the same time, we have acted in a deliberate manner that aims to deescalate the overall situation in eastern Syria and Iraq.”
Kirby said the US airstrikes “destroyed multiple facilities at a border control point used by a number of Iranian- backed militant groups," including Kataib Hezbollah and Kataib Sayyid al-Shuhada.
The US has blamed Kataib Hezbollah for numerous attacks targeting US personnel and interests in Iraq in the past.
Further details were not immediately available.
Mary Ellen O'Connell, a professor at Notre Dame Law School, criticized the US attack as a violation of international law.
“The United Nations Charter makes absolutely clear that the use of military force on the territory of a foreign sovereign state is lawful only in response to an armed attack on the defending state for which the target state is responsible,” she said. “None of those elements is met in the Syria strike.”
A little-known Shiite militant group calling itself Saraya Awliya al-Dam, Arabic for Guardians of Blood Brigade, claimed responsibility for the February 15 attack.
A week later, a rocket attack in Baghdad's Green Zone appeared to target the US Embassy compound, but no one was hurt.
Iran this week said it has no links to the Guardians of Blood Brigade.