The United States-led airstrikes on Thursday and Friday against sites in Yemen controlled by the Houthi militia damaged or destroyed about 90 percent of the targets struck, but the group retained about three-quarters of its ability to fire missiles and drones at ships transiting the Red Sea, two U.S. officials said on Saturday.

The damage estimates are the first detailed assessments of the strikes by American and British attack planes and warships against nearly 30 locations in Yemen, and they reveal the serious challenges facing the Biden administration and its allies as they seek to deter the Iran-backed Houthis from retaliating, secure critical shipping routes between Europe and Asia, and contain the spread of regional conflict.

A top U.S. military officer, Lt. Gen. Douglas Sims, the director of the military’s Joint Staff, said on Friday that the strikes had achieved their objective of damaging the Houthis’ ability to launch the kind of complex drone and missile attack they had conducted on Tuesday.

But the two U.S. officials cautioned on Saturday that even after hitting more than 60 missile and drone targets with more than 150 precision-guided munitions, the strikes had damaged or destroyed only about 20 to 30 percent of the Houthis’ offensive capability, much of which is mounted on mobile platforms and can be readily moved or hidden.

The two U.S. officials spoke on the condition of anonymity to discuss internal military assessments.

Finding Houthi targets is proving to be more challenging than anticipated. American and other Western intelligence agencies have not spent significant time or resources in recent years collecting data on the location of Houthi air defenses, command hubs, munitions depots and storage and production facilities for drones and missiles, the officials said.

That all changed after the Hamas attacks on Israel on Oct. 7, and the Israeli military’s responding ground campaign in the Gaza Strip. The Houthis have been attacking commercial ships transiting the Red Sea in solidarity with Palestinians in Gaza, and have said they will continue until Israel withdraws. U.S. analysts have been rushing to catch up and catalog more potential Houthi targets every day, the officials said.

Thursday night’s air and naval barrage illustrated this approach, military officials said. The first wave of U.S.-led strikes hit 60 preplanned targets in 16 locations with more than 100 precision-guided bombs and missiles. About 30 to 60 minutes after that, a second wave of strikes was carried out against 12 more targets that analysts had identified as posing threats to aircraft and ships.

Hitting pop-up targets on short notice, a practice the military calls dynamic targeting, would likely be an important part of any additional strikes that President Biden might order, one of the U.S. officials said.

A senior Defense Department official said on Saturday that a U.S. Tomahawk cruise missile strike on a radar facility in Yemen on Friday was a “reattack” of a target originally hit in Thursday’s barrage that had not been adequately degraded or destroyed.

Other U.S. military officials said that as analysts review the damage from Thursday night’s airstrikes, there may be additional reattacks.

Despite their fiery rhetoric and vows of retaliation, the Houthis’ military response to Thursday night’s attack so far has been muted: just a single anti-ship missile lobbed harmlessly into the Red Sea, far from any passing vessel, General Sims said on Friday.

But the general and the two U.S. officials on Saturday said they were bracing for the Houthis to lash out once they determined how much firepower they had left and settled on an attack plan.

One of the two U.S. officials said the Houthis appeared to be divided internally over how to respond.

“I would expect that they will attempt some sort of retaliation,” General Sims said on Friday, adding that that would be a mistake. “We simply are not going to be messed with here.”