Dozens have been killed and some 54,000 civilians have fled Abyan, which has descended into daily bloodshed as the army confronts militants the government says have ties to al Qaeda.

The region lies east of the strategic Bab al-Mandab strait, where some 3 million barrels of oil pass daily.

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.

"The head of the Defence Ministry sent reinforcements including tanks, rocket launchers and 500 extra soldiers," a local official said.

"These forces began attacking (the city) backed by heavy tank shelling and rocket attacks from naval ships in order to liberate the 25th Brigade just outside Zinjibar and under siege for over a month."

Residents said dozens were hurt on both sides in street fighting, after troops and tribesmen entered the city from the east.

A local official said three militant leaders were killed, including Nasser al-Maraji, whom he identified as a prominent local Islamist leader.

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.

On Sunday, troops loyal to Saleh opened fire to disperse a protest march in the Red Sea port city of Hudaida, residents said. A hospital official said about 50 people were injured.

Saleh is convalescing in the Saudi capital Riyadh after being injured by an attack on his presidential compound.

The United States and oil giant Saudi Arabia are keen to stem the chaos in Yemen, fearing the growing power vacuum gives extra room to al Qaeda's regional wing. Both countries have been targets of failed al Qaeda attacks from Yemen.

Tribesmen who joined the offensive said they had sent about 450 men to Zinjibar. They had begun to plan attacks on the militants last week, saying the army had been ineffective.

The heavy offensive, which began late on Saturday, has caused dozens of casualties in Zinjibar, residents told Reuters by telephone, describing how army ambulances screeched through the city on Sunday, filled with dozens of wounded people.

In nearby Jaar, Islamist militants who seized the city in March sent gunmen to surround and occupy a government hospital, medics at the hospital told Reuters.

The militants were now using the hospital to treat their wounded fighters from Zinjibar, they said. Doctors and patients were permitted to leave the hospital, they said, as the militants brought their own medical team into the hospital.

A local official earlier told Reuters some 20 militants were killed and dozens on both sides were injured during the fighting. He said 35 militants had been killed since the offensive began, but only confirmed the death of two soldiers.

Medical workers in Zinjibar declined to give an estimate of soldiers' deaths, saying they were too overwhelmed with casualties entering the hospital.

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.

Militants who seized Jaar in March and took Zinjibar in May had taken control of a football stadium outside Zinjibar in June, which the army had been using as a makeshift supply base.

Troops had been fighting militants around the soccer field since dawn, residents and a local official said, and armoured vehicles shelling the area destroyed part of the stadium.