The United States and Britain carried out another round of large-scale military strikes Saturday against multiple sites in Yemen controlled by Houthi militants, U.S. officials said.

The strikes were intended to degrade the Iran-backed militants’ ability to attack ships in sea lanes that are critical for global trade, a campaign they have carried out for almost four months.

American and British warplanes hit missile systems and launchers and other targets, the officials said. Australia, Bahrain, Canada, Denmark, the Netherlands and New Zealand provided support for the operation, according to a joint statement from the countries involved that was emailed to reporters by the Defense Department.

The strikes, which the statement called “necessary and proportionate,” hit 18 targets across eight locations in Yemen associated with Houthi underground weapons storage facilities, missile storage facilities, one-way attack unmanned aerial systems, air defense systems, radars and a helicopter.

“These precision strikes are intended to disrupt and degrade the capabilities that the Houthis use to threaten global trade, naval vessels, and the lives of innocent mariners in one of the world’s most critical waterways,” the statement said.

The strikes were the largest salvo since the allies struck Houthi targets on Feb. 3 and came after a week in which the Houthis have launched attack drones and cruise and ballistic missiles at vessels in the Red Sea and the Gulf of Aden.