U.S. forces followed up strikes on Syria and Iraq with a new round of attacks Saturday against Iran-backed Houthis in Yemen, signaling that President Joe Biden is willing to deepen U.S. involvement in the region’s spreading conflict.

Targeting Houthi sites at 13 locations in Yemen, the U.S. and its allies escalated their effort to destroy the militant group’s ability to attack Red Sea shipping with strikes including their hardened facilities. It marked the biggest set of strikes against the Houthis since an initial operation on Jan. 11, also carried out jointly by the U.S. and the U.K.

Supported by six other countries including Bahrain, the strikes on Saturday targeted 36 Houthi sites in all, specifically those associated with the group’s deeply buried weapons storage facilities, missile systems and launchers, air defense systems and radars, the Pentagon said in a statement.