The United States and Britain on Saturday struck dozens of targets in Yemen in response to repeated attacks on shipping in the Red Sea and Gulf of Aden by Iran-backed Huthi rebels.

Those strikes came the day after a separate wave of unilateral American strikes against Iran-linked targets in Iraq and Syria that were carried out in response to the killing of three US soldiers in Jordan on January 28.

"That is not the end of it. We intend to take additional strikes and additional action to continue to send a clear message that the United States will respond when our forces are attacked," Sullivan told NBC news Sunday morning.

He also told ABC that the strikes have had a "good effect" in "reducing" and "degrading" such groups' abilities to carry out further attacks.

The United States has carried out a total of 85 strikes on seven separate sites in Iraq and Syria, including on command and training centers and on weapons stocks, according to the Pentagon.

Sullivan said Sunday he was unable to confirm whether the strikes had left any civilian casualties, but said the targets were "absolutely valid."

Asked whether the United States would rule out the possibility of striking Iran directly, Sullivan told NBC, "Sitting here on television, it would not be wise for me to talk about what we're ruling in and ruling out."

"If (Iran) chose to respond directly to the United States, they would be met with a swift and forceful response from us," he told ABC.

The Iran-backed Huthis began targeting Red Sea shipping in November, saying they were hitting Israel-linked vessels in support of Palestinians in Gaza, which has been ravaged by the Israel-Hamas war.