President Joe Biden said Friday that a US airstrike against an Iranian-backed militia in eastern Syria should be seen by Iran as a warning.

Asked what the message was from the air strike, Biden said: "You can't act with impunity."

"Be careful," he added, speaking in Houston during a tour of relief efforts after a huge winter storm in Texas.

Earlier, Biden's press secretary, Jen Psaki, called the raid an "unambiguous message" that Biden is "going to act to protect Americans and when threats are posed he has the right to take an action at the time and the manner of his choosing."

The Pentagon said Thursday's strike, which according to a Syrian war monitoring group killed 22 militia members, was in response to a series of rocket attacks targeting US soldiers in Iraq.

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.

Psaki said the decision behind the strike was "deliberative" and that Biden's aim was for "deescalating activity in both Syria and Iraq."

Pentagon spokesman John Kirby said two F-15E "Strike Eagles" dropped seven precision-guided munitions, totally destroying nine facilities and partially destroying two facilities.

Kirby said the location targeted near the Syria-Iraq border was "known to facilitate Iranian-aligned militia group activity."

He said the Pentagon had "preliminary details about casualties" but declined to release any figures.

"We're confident that these were legitimate targets that were utilized by groups associated with these recent attacks," Kirby said.

He said Iraqi and Kurdish partners had provided intelligence that led to the identification of the groups behind the rocket attacks.

Kirby also responded to criticism by Russian Foreign Minister Sergei Lavrov, who said Moscow was notified just four or five minutes before the US struck the targets.

"We did what we believe was the proper amount of notification for this," Kirby said. "It shouldn't come as a shock to anybody that we're going to do what we have to do to notify but we're also going to do what we have to do to protect our forces."

Kirby and Psaki also addressed criticism from some members of Congress that Biden should have sought legislators' authority before ordering the strike.

Psaki said the Pentagon briefed congressional leadership "before the action."

"There will be a full classified briefing early next week at the latest," she said.