The United States struck two Houthi anti-ship missiles in Yemen, the military’s Central Command said on Tuesday, resuming what U.S. officials said were short-notice attacks against the Iran-backed militia’s imminent threats to merchant vessels, as well as Navy ships in the Red Sea and nearby waters.

The U.S. strikes — the ninth in two weeks — came a day after the United States and Britain carried out much larger military salvos against nine sites in Yemen controlled by the Houthis. Those strikes against multiple targets at each location hit radars, as well as drone and missile sites, and underground weapons storage bunkers.

While the Pentagon points to the fact the Houthis haven’t mustered an attack against any ships in the Red Sea or Gulf of Aden since Jan. 18 as evidence that the spot-and-shoot American strikes are wearing down the rebels’ formidable missile and drone arsenal, other analysts fear that the Houthis are not really deterred and are just biding their time.

President Biden has signaled his approval for a sustained, if limited, air campaign, blessing a Pentagon strategy to put armed Reaper drones and other surveillance platforms up in the skies over Yemen, so that U.S. warplanes and ships can hit Houthi mobile targets as they pop up.

“Are they stopping the Houthis? No,” Mr. Biden said of the airstrikes last week. “Are they going to continue? Yes.”