Yemen’s Iran-backed Houthi rebels captured more strategic islands in the southern Red Sea, government and Houthi officials said Monday, strengthening their control of a major maritime shipping route.

The capture of the islands of Greater and Lesser Hanish was the latest in the rebels’ swift advance around the Bab el-Mandeb Strait, a key passage for Saudi oil shipments that has only become more crucial during the Iran war since it serves as an alternative to the Strait of Hormuz.

Like Hormuz, which is located at the mouth of the Persian Gulf, the Bab el-Mandeb Strait — on the opposite side of the Arabian Peninsula — is a chokepoint that connects the Red Sea to the open ocean and Saudi Arabia’s key Asian markets.

For more than a month, the Houthis have been striking Saudi oil infrastructure and shipping in the Red Sea, stepping up pressure on global oil prices and boosting Iran’s leverage in its war with the United States.

Saudi-backed Yemeni government forces have been attempting to rally and fight back. But the Houthi gains have seemingly come with little resistance.