Dozens have been killed and some 54,000 civilians have fled Abyan, which has descended into daily bloodshed as the army faces a rising challenge from militants who the government says have ties 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, through which some 3 million barrels of oil pass daily.

"The head of the Defense 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."

Tribesmen who joined the weekend 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 Saturday night, 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, 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.

In March, militants seized the city of Jaar, also in Abyan province, and later took Zinjibar in May. They took control of a football stadium outside Zinjibar in June, which the army had been using as a makeshift supply base.

Army units have been fighting militants around the football field since dawn, residents and a local official said, and armored vehicles shelling the area have destroyed part of the stadium.