The U.S. military launched airstrikes that destroyed three anti-ship cruise missile launchers in Houthi-controlled Yemen late on Thursday, as the armed rebel group stepped up its attacks on ships in the Red Sea, the U.S. Central Command said.

The latest strikes from U.S. forces came as the Houthis, an Iranian-backed group that controls much of Yemen, intensified their campaign in the Red Sea, firing missiles toward two ships on Thursday after crippling a vessel the previous day, maritime security monitors said.

One merchant vessel was hit by projectiles about 98 nautical miles east of the Yemeni city of Aden and caught fire, the United Kingdom Maritime Trade Operations, a British agency, said in an online post. It did not name the ship.

But the U.S. Central Command said two Houthi missiles had struck a Ukrainian-owned bulk carrier, the Verbana, in the Gulf of Aden, severely injuring a crew member, who was later evacuated to another ship.

The Polish-operated vessel was en route to Italy carrying lumber and caught fire. “The crew continues to fight the fire,” the U.S. military said.

The British maritime agency said it also received a report from an officer of a second ship, about 82 nautical miles northwest of the port of Hodeida, that there was an explosion near the vessel.

“There was no damage to the vessel, all crew are reported safe and the vessel is proceeding to its next port of call,” the agency said.

A Houthi military spokesman, Yahya Saree, said in a televised speech that in addition to the Verbana, they also struck two more vessels, which he identified as the Seaguardian and the Athina. His claim could not be independently verified.

On Wednesday, a Greek merchant vessel, the Tutor, sent out a distress call after the Houthis said they targeted the ship using unmanned surface boats, a number of drones and ballistic missiles.

The Central Command said the impact of the attack caused severe flooding and damage to the engine room. The fate of the ship remained unclear on Thursday. The “continued malign and reckless behavior” by the Houthis threatens regional stability and endangers the lives of mariners across the Red Sea and Gulf of Aden, the Central Command said.

The threat posed by the Houthis in the Red Sea makes it harder to deliver critical assistance to the people of Yemen as well as to Gaza, it added.