The statement, which was accompanied by a call from Hillary Clinton, the US secretary of state, for "immediate transition", dealt a significant blow to loyalist hopes that Ali Abdullah Saleh would resume office after a period of convalescence in a Saudi Arabian hospital.
As he recovered from two operations to remove shrapnel and reconstruct his skull, Mr Saleh's hold on power slipped further on Tuesday after fighters from the Sharab and Same tribes, both led by senior military figures, took control of most of Taiz, Yemen's second city.
Unless pro-regime forces stage a major fightback, Mr Saleh, 69, if he were to return, faces the prospect of a Libya-style situation in which the government controls only a portion of the country with revolutionary forces holding sway in cities such as Taiz.
Senior officials in Mr Saleh's government, including the vice-president, have insisted that he will return to Yemen within days.
But it is now thought likely that Mr Saleh will not recover for months.
He was wounded, along with a number of senior officials, in an attack last Friday on his palace.
His injuries were initially thought to be caused by shellfire but US officials said the attack was a bomb, pointing to insider involvement.
