U.S. fighter jets struck Iranian-backed Houthi rebel sites for the sixth time Friday, taking out three anti-ship missiles in Yemen that were prepared to fire, according to U.S. officials.

The officials, who spoke on condition of anonymity to discuss ongoing military operations, said the strikes were carried out by F/A-18 aircraft off the USS Dwight D. Eisenhower aircraft carrier. And they resembled similar U.S. attacks on Houthi launchers that have been occurring almost daily this week.

In a statement later Friday, U.S. Central Command said the strike was at about 6:45 p.m. local time in Sanaa, Yemen’s capital, and that the missiles were aimed into the southern Red Sea and were prepared to launch. It said the U.S. determined they presented an imminent threat to merchant vessels and the U.S. Navy ships in the region, and so struck them in self-defense. The statement did not say how the strikes were carried out.

President Joe Biden acknowledged Thursday that the bombardment of Houthi sites, including a massive array of strikes on Jan. 12 by U.S. and British forces, has yet to stop the militants’ attacks on vessels in the Red Sea that have disrupted global shipping.

Al-Masirah, a Houthi-run satellite news channel, said there were air raids in the western city of Hodieda on Friday, targeting the al-Jabaana neighborhood in the west of the city. The location of the U.S. strikes could not be immediately confirmed.

U.S. warships and aircraft, in rapid succession, have taken out Houthi missiles poised to launch over the past few days, underscoring the military’s increasing ability to watch, detect and strike militant activities in Yemen.

At the White House, National Security Council spokesman John Kirby noted the uptick in preemptive missions.

“This is the fourth preemptive action that the U.S. military has taken in the past week against Houthi missile launchers that were ready to launch attacks, in this case, anti-ship missiles,” he said, adding that the self-defense strikes are aimed at improving security in the shipping lanes.

But so far the strikes have not deterred Houthi attacks on ships in the southern Red Sea or Gulf of Aden, which also have been happening nearly daily.

The Biden administration put the Houthis back on its list of specially designated global terrorists. The sanctions that come with the formal designation are meant to sever violent extremist groups from their sources of financing, while also allowing vital humanitarian aid to continue flowing to impoverished Yemenis. And the White House has made it clear that the retaliatory strikes will also continue.

“They continue to have offensive capability, and they still continue to be willing to use it,” Kirby said. “We also have plenty of defense capability available to us and we continue to use it as well.”

For months, the Houthis have attacked ships in the Red Sea that they say are either linked to Israel or heading to Israeli ports. They say their attacks aim to end the Israeli air-and-ground offensive in the Gaza Strip that was triggered by the Palestinian militant group Hamas’ Oct. 7 attack in southern Israel. But the links to the ships targeted in the rebel assaults have grown more tenuous as the attacks continue.