President Abed Rabbo Mansour Hadi "cannot leave his house" after Houthi rebels removed his guards and deployed their own fighters there today, one aide said.
Another described the country being at the "point of no return." Hadi can't resign as president as the Houthis have threatened to prosecute him, that aide said. The two aides spoke on condition of anonymity as they weren't authorised to speak to journalists.
The embattled Hadi, a top US ally in the war against al-Qaida in Yemen, appears to have run out of options to continue governing the country, months after the Houthis began a blitz across the country in September.
Advisers said today that Houthis issued a list of demands to Hadi, asking for the post of vice president and several key government offices, during a meeting yesterday with Hadi's advisers.
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The collapse of Hadi's powers is rooted in Yemen's fractured armed forces, torn between Hadi and his predecessor, deposed president Ali Abdullah Saleh.
Saleh, removed from power after 2011 uprising, is accused by many as orchestrating Houthis' seizure of Sanaa and speeding Hadi's failure.
Capitalising on the chaos, Saleh made a rare public statement today, urging Hadi to call for early presidential and parliamentary elections.
Saleh also called for the cancellation of United Nations Security Council sanctions imposed on him and two Houthi leaders last year for "spoiling" the transition.
Saleh was granted immunity from prosecution in 2012 in return for relinquishing power in a deal brokered by Arab Gulf countries and backed by the West.
The Houthis are a political and religious movement who are followers of the Shiite sect of Zaydism, who make up a third of Yemen's population of 25 million people and live mainly in the north. Sunnis live mostly in the country's south and make up two thirds of the population.