Heavy clashes have rocked northern neighbourhoods of Yemen's capital Sanaa, breaking a truce aimed at ending the worst violence since a popular revolt against President Ali Abdullah Saleh began eight months ago.

A reporter at the scene said three areas in north Sanaa had been hit by heavy shelling and gunfire between government troops and armed followers of opposition leader Sadeq al-Ahmar.

Many residents fled their homes this morning as the fighting intensified, shattering three days of calm in the capital after Mr Saleh ordered a ceasefire upon his surprise return to Yemen last week.

The truce followed more than a week of fighting in which over 100 people were killed, raising worries that the country could be dragged closer to civil war.

Mr Saleh had been recuperating in Riyadh for three months after a bomb attack in June and had been pressured by Western diplomats to stay in Saudi Arabia while they struggled to push through a long-stalled power transition plan.

The president has faced the biggest challenge to his 33-year rule in mass protests across the country demanding his overthrow.

Powerful figures once close to Mr Saleh have supported the protests.

Sanaa is now carved up into different spheres of influence of government troops and pro-opposition forces.

During the lull politicians and diplomats had tried to get negotiations back on track on a plan brokered by Gulf Arab states under which Mr Saleh would stand down.