The United States will keep attacking Yemen's Houthis until they end attacks on shipping, the U.S. defense secretary said on Sunday, as the Iran-aligned group signaled it could escalate in response to deadly U.S. strikes the day before.

The airstrikes, which the Houthi-run health ministry said killed at least 53 people, are the biggest U.S. military operation in the Middle East since President Donald Trump took office in January. One U.S. official said the campaign might continue for weeks.

Houthi leader Abdul Malik al-Houthi said on Sunday that his militants would target U.S. ships in the Red Sea as long as the U.S. continues its attacks on Yemen.