A suicide bomber in army uniform killed more than 90 soldiers in the heart of the Yemeni capital on Monday and an Al Qaeda affiliate threatened more attacks if a US-backed campaign against militants in the front-line state did not stop.

The bombing, which wounded more than 200 people, underscored the dangers Yemen faces as it battles Islamist militants entrenched in the south and threatening shipping lanes in the Gulf of Aden and the Red Sea.

It left scenes of carnage in Sanaa's Sabaeen Square, where the military had been rehearsing for a parade. Body parts lay strewn across a 10-lane road not far from the presidential palace.

'We had just finished the parade. We were saluting our commander when a huge explosion went off,' said soldier Amr Habib. 'It was a gruesome attack. Many soldiers were killed and others had their arms and legs blown off.'

US President Barack Obama's counter-terrorism adviser, John Brennan, called Yemen's President Abd-Rabbu Mansour Hadi to convey Washington's condemnation of the attack and pledge support for Yemen, a major front in its global war on al Qaeda.

Hadi, who took over after Washington and its Saudi-led Gulf allies persuaded his predecessor to step down to prevent an uprising spreading anarchy in Saudi's neighbour, vowed to step up the fight against militant strongholds in the south.

'The war on terrorism will continue until it is uprooted and annihilated completely, regardless of the sacrifices,' Hadi said, according to a text published by the state news agency.

One investigator said preliminary findings suggested the bomber was a rogue soldier who had somehow evaded security checks rather than a man in a disguise.

'The suicide bomber was dressed in a military uniform. He had a belt of explosives underneath,' said a man who identified himself as Colonel Amin al-Alghabati, his hands and uniform flecked with blood.

The defence ministry said at least 90 soldiers had been killed and 222 people wounded. The al Qaeda affiliate said it had targeted the defence minister and army commanders.

The defence minister and chief of staff were both at the rehearsal for Tuesday's National Day parade - meant to celebrate Yemeni unity - but neither was hurt. Yemeni officials said Hadi had moved Tuesday's parade to a military academy.-Reuters