SAN’A, Yemen—Yemeni officials said that Shiite rebels have captured a key military base south of the capital, where U.S. advisers had previously trained counterterrorism forces.

They say that the Republican Guard camp captured on Thursday was used by American experts until 2012 to train local forces battling al Qaeda’s powerful Yemeni affiliate. The officials spoke on condition of anonymity because they were not authorized to speak to the press.

The Shiite rebels, known as Houthis, have seized a number of key military facilities in recent days, including the headquarters of the paramilitary special forces. They took control of the capital in September.

The captured camp was manned by forces loyal to former President Ali Abdullah Saleh, who is suspected of quietly backing the rebels.