SANAA, Yemen -- Yemen's Shiite rebels took over a key military base south of Sanaa where U.S. advisers had previously trained counter-terrorism forces, officials said Thursday.

Military officials said the Republican Guard camp captured Wednesday was used by American experts until 2012 to train local forces battling al-Qaida'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.

Last week the rebels put President Abed Rabbo Mansour Hadi, a close U.S. ally, and his Cabinet under house arrest, escalating a prolonged power struggle. The officials resigned in response, and Prime Minister Khaled Baha said Thursday his resignation was "irreversible" according to a statement posted on his Facebook account.

Baha expressed his "extreme condemnation" over the Houthis' "coup," adding that he had escaped an assassination attempt. He said he would refuse to serve in a caretaker government.

Anti-Houthi demonstrations meanwhile erupted in Sanaa, with rebel forces dispersing the protesters with batons and shots fired in the air. Protesters attempted to break the Houthis' siege on the home of the minister of local administration but the rebels forcibly drove the crowd away.

Larger demonstrations were held in the western port city of Hodeida -- which is controlled by the Houthis -- and Taiz, where scuffles broke out between demonstrators and rebels.

The camp captured Thursday was led by forces loyal to former President Ali Abdullah Saleh, who is suspected of quietly backing the rebels. The seizure of the camp could drive a wedge between the two, fueling further turmoil.

In the central city of Houta, meanwhile, extremists on motorbikes destroyed the 800-year-old shrine of a Muslim scholar, according to a local security official. Hard-line Muslims oppose the veneration of saints, saying it goes against monotheism. Salafis in Mali, Somalia and neighbouring Libya have all targeted the tombs of saints.