Yemen’s Houthi fighters hijacked a United Arab Emirates-flagged cargo ship off the coast of Yemen on Sunday night, according to a Saudi Arabia-led military coalition that’s battling the rebel force.

The vessel, Rawabi, was carrying supplies used to operate a Saudi hospital on the island of Socotra, and was sailing near the Red Sea port city of Hodeida when it was attacked, the coalition said in a statement on Monday. The coalition demanded that the fighters “immediately release the ship” and threatened to use force if they didn’t.

Houthi armed forces spokesman Yahya Saree said in a statement that their fighters had captured a military cargo ship carrying weapons after it entered Yemen territory without permission.

The Houthis have been fighting Yemen’s United Nations-recognized government since 2014. A Saudi Arabian-led coalition intervened the following year with a bombing campaign, on the side of the government. The UN has called the conflict—in which tens of thousands of people have died—the world’s worst humanitarian crisis.