The cost of insuring commercial ships that sail past Yemen’s Houthi militants surged after the rebels sank two ships and killed sailors this week, underscoring how the attacks have revived risk in the crucial waterway.

The U.S. military spent hundreds of millions of dollars earlier this year in a six-week barrage intended to stop the Iran-backed Houthi rebels in Yemen from targeting ships in the Red Sea.

Crew members aboard a Liberian-flagged ship set ablaze by a series of attacks in the Red Sea have abandoned the vessel as it took on water.