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

The attacks were aimed at degrading the Iranian-backed militants’ ability to attack ships in sea lanes that are critical to global trade, a campaign they have waged for nearly four months.

American and British fighter jets attacked missile systems, launchers and other targets, the officials said.

The attacks were the largest salvo since the allies attacked Houthi targets on February 3 and came after a week in which the Houthis launched attack drones and ballistic and cruise missiles against vessels in the Red Sea and Gulf of Aden.

On Monday, Houthi militants fired two anti-ship ballistic missiles at a cargo ship, the US Central Command said in a statement. The ship, named Sea Champion, continued towards its destination at the port of Aden in Yemen, the statement added. Central Command reported several other retaliatory attacks that day between US forces in the area and the Houthis.

Thursday was more of the same. U.S. warplanes and a ship belonging to a member of the U.S.-led coalition shot down six Houthi attack drones in the Red Sea, Central Command said in a separate statement. The drones were “likely targeting US and coalition warships and were an imminent threat,” he added.

Later that day, according to the statement, the Houthis fired two anti-ship ballistic missiles from southern Yemen into the Gulf of Aden, hitting the Islander, a British-owned cargo ship flying the Palau flag. The boat was damaged and one person suffered minor injuries.

And earlier on Saturday, the naval destroyer USS Mason shot down what Central Command said was an anti-ship ballistic missile launched from Yemen toward the Gulf of Aden.

The Houthis say their attacks are a protest against Israel’s military campaign in Gaza, which was launched in response to Hamas attacks in Israel on October 7.

The U.S.-led air and naval strikes began last month in response to dozens of Houthi drone and missile attacks on commercial shipping in the Red Sea since November.

The United States and several allies repeatedly warned the Houthis of serious consequences if they did not stop the bombing. But US-led strikes have so far failed to deter the Houthis. Hundreds of ships have been forced to take a long detour around southern Africa, driving up costs.

Of all the Iranian-backed militias that had escalated hostilities in solidarity with the Palestinians in Gaza, the Houthis have been perhaps the most difficult to contain. While the Houthis have continued their attacks, Shiite militias in Iraq and Syria appear to be observing a period of calm since the United States carried out a series of attacks against Iranian forces and the militias they support in Syria and Iraq on 2 of February.

Middle East experts say that after nearly a decade of evading airstrikes in a war with Saudi Arabia, the Houthis have become adept at hiding their weaponry, placing some of it in urban areas and firing missiles from the rear of the vehicles before escaping.

By Sam