"We announce that we support and protect the youths who are protesting at University Square in Sanaa," the armoured infantry division commander told reporters.

"The crisis is getting more complicated and it's pushing the country towards and violence and civil war.

"According to what I'm feeling, and according to the feelings of my partner commanders and soldiers... I announce our support and our peaceful backing to the youth revolution.

"We are going to fulfil our duties in preserving security and stability."

Al-Ahmar is the most senior military officer to pledge support for the opposition, which has been agitating for weeks to end Saleh's 32-year rule over the impoverished, tribal country.

His pledge, made in a statement broadcast on Al-Jazeera television, comes a day after Saleh sacked his entire cabinet in a bid to placate opposition calls for sweeping reforms.

The regime has already been weakened by the resignations of three ministers, several ambassadors and a host of ruling party MPs, but Saleh has refused to stand down until his term ends in 2013.

His regime was internationally condemned on Friday when more than 50 people were killed as loyalist gunmen opened fire on protesters in Sanaa's University Square, the centre of the pro-democracy movement.

The defection of top military officers to the opposition is likely to complicate Washington's support for Saleh, whom it sees as a pillar of stability in a volatile country and a partner in the war against Al-Qaeda.