"I would hope that the message sent by the strikes last night will be heard and deter future action," Blinken told reporters on a visit to Rome.

"This action in self-defence to do what's necessary to prevent further attacks sends a very important and strong message," he said.

The Pentagon announced that three military facilities used by Iran-backed militia had been hit overnight Sunday to Monday -- two in Syria and one in Iraq.

It said the targets had been used by "Iran-backed militias that are engaged in unmanned aerial vehicle (UAV) attacks against US personnel and facilities in Iraq".

Asked in Rome if the United States was holding Iran responsible for the attacks, Blinken said: "A number of the groups involved in recent attacks are militia that are backed by Iran."

The air strikes on the Iraq-Syria border area were the second attacks ordered by President Joe Biden in response to strikes on US forces in Iraq.

The attacks were condemned by Iraqi Prime Minister Mustafa al-Kadhemi, who called them a violation of his country's sovereignty.