Yemen’s Houthis captured the Hanish archipelago in the southern Red Sea, strengthening their control of a major maritime shipping route after hundreds of government-allied forces withdrew, government and Houthi officials said on Monday.

The group officially known as Ansar Allah deployed on the islands of Greater and Lesser Hanish, located 160 kilometres north of the Bab el-Mandeb Strait, adding to their capture of the nearby mainland port city of Mokha and the island of Perim. Forces of the internationally recognised Yemeni government have been attempting to rally and fight back, but the Houthi gains have seemingly come with little resistance.

More than 80,000 people have been displaced in two weeks of fighting, as the internationally recognised government vows to regroup following the swift advance.

Addressing the developments, US Vice President J.D. Vance told reporters at Joint Base Andrews in Maryland on Thursday that the United States is “concerned” about Houthi movements in the Red Sea.

Vance stated that Washington is “monitoring the situation closely” and is “in continuous contact with Saudi Arabia, the UAE, and other affected countries in the region.”

The U.S. administration is “engaged in talks with the Houthis, and we feel we are in control of this constantly changing situation, and are studying it well to ensure American interests,” Vance added.

The Bab el-Mandeb Strait—located on the opposite side of the Arabian Peninsula from the Strait of Hormuz—is a chokepoint that connects the Indian Ocean to the Red Sea, making it a key waterway for cargo traffic to the Mediterranean Sea. The passage is a key route for oil shipments and has become even more crucial during the Iran war, as the Strait of Hormuz, located at the mouth of the Persian Gulf, remains under U.S. and Iranian blockades.

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 U.S.