Two other senior army commanders are also reported to have resigned. President Saleh said he was "holding out" and the National Defence Council said it would block any "coup".

Tanks were deployed in the capital, Sanaa, apparently to defend key points including the presidential palace, defence ministry and central bank. One by one, the pillars of President Saleh's power are being knocked away.

The military commanders who announced their resignations are from President Saleh's Hashid tribe.

Sadiq al-Ahmar, head of the Hashid tribal federation, told al-Jazeera that it was time for Mr Saleh to make a "quiet exit".

One of the tribe's leaders said the Hashid were rallying behind Gen Ahmar as a possible replacement for President Saleh, the Associated Press reported.

Meanwhile, Yemen's foreign minister was sent to Saudi Arabia with a message from President Saleh to King Abdullah, the state news agency said. The contents of the message were not reported.

Yemen is one of a number of countries in the region that have seen unrest since the presidents of Egypt and Tunisia were ousted in popular revolts.