Four soldiers and ten militants linked to al-Qaeda have been killed in clashes in the southern Yemeni town of Zinjibar.

Yemeni forces, backed by armed tribesmen, launched an offensive to retake Zinjibar, capital of southern Abyan province, after months of fighting with Islamist militants who seized the capital and another city.

54,000 civilians have fled Abyan, which has descended into daily bloodshed as the army faces a rising challenge from militants the government says have ties to al-Qaeda.

The fighting since last night centred around the base of the 25th Mechanised Brigade, near which troops found and destroyed an arms cache belonging to elements linked to al-Qaeda.

After weeks of pleas for support from a besieged military brigade near Zinjibar, the government sent the first reinforcements on Saturday, aiming to flush militants out of the seaside city.

Zinjibar lies east of the strategic Bab al-Mandab strait, where some 3m barrels of oil pass daily.

While unrest mounts in Abyan, mass protests demanding President Ali Abdullah Saleh leave office have entered their sixth month, paralysing several cities and pushing the country into political limbo.

Saleh is convalescing in the Saudi Arabian capital Riyadh after being injured by a bomb blast on his presidential compound.

Opposition groups accuse Saleh of letting his forces ease up in the south to stoke fears in the international community that only he stood in the way of a militant takeover.