The U.S. and U.K. conducted large-scale air and missile strikes on Houthi rebel facilities across Yemen on Monday, according to a U.S. and British official, stepping up operations against the militant group as it vows to continue attacking ships in the Red Sea.

The U.S. military has launched multiple rounds of preemptive strikes against Houthi anti-ship missile sites that were preparing to fire against international shipping over the past week. But those strikes have failed to deter the militants from continuing to attack commercial vessels transiting the Red Sea and Gulf of Aden.

This is the second time U.S. and U.K. forces have conducted joint air and missile strikes on the Houthis in Yemen since the group began attacking international shipping in November. The two militaries, with support from Australia, the Netherlands, Canada and Bahrain launched strikes for the first time on Jan. 11, after the Houthis ignored weeks of warnings by Washington and its allies to stop the attacks.

Since then, the U.S. has hit Houthi targets a half-dozen times on its own, but those strikes were conducted against what the Pentagon has characterized as an “imminent” threat and were smaller in scale.

President Joe Biden and his top aides have repeatedly said strikes on Houthi targets alone won’t stop the Iran-backed militants from threatening commercial ships in the Middle East.

Administration officials say it could take weeks, maybe months, to apply the necessary military, economic and diplomatic pressure to force the Houthis to change course.

An end to the war between Israel and Hamas, or at least a slowdown in fighting, could weaken the Houthis’ case for endangering global shipping in regional waters.