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By Sahal Abdulle

MOGADISHU, March 31 (Reuters) - Shelling rocked Mogadishu for a third day on Saturday, overwhelming hospitals with casualties as Ethiopian and Somali troops backed by helicopter gunships attacked Islamist rebels and clan militia.

Scores of civilians have been killed in what the International Committee of the Red Cross says is the capital's worst fighting for more than 15 years.

Ethiopia said its military had killed more than 200 "armed remnants" of a hardline Islamist movement ousted from Mogadishu in a war over the New Year. Terrified residents said volleys of artillery rounds began crashing down hours before dawn.

"We ran to help our neighbours when a shell hit their house, then suddenly another one hit us, wounding me and four of my children. Only my 14-month-old is okay," mother-of-five Samsam Mukhtar said outside a hospital, her eyes filling with tears.

Others said the city was being blasted indiscriminately.

"Whoever is doing this is not human," Salado Yebarow, who lives between the main stadium and the presidential palace, told Reuters by telephone. "They have clearly never had a grandmother or children to think about."

Hospitals struggled to cope with injured civilians, even though most victims could not reach any kind of help because of ongoing battles. Doctors were also trapped by the fighting.

At the city's main Madina Hospital, many patients lay on thin mattresses in the yard. Others wailed inside packed wards.

NO RELIEF

"I have never seen anything like this," hospital director Sheikhdon Salad Elmi told Reuters. "We are operating with only half our surgeons here, and the doctors who are here have now been working without relief for the last three days."

Thousands of people have fled the city in recent days, and a Reuters reporter said thousands more took to the streets on foot at first light on Saturday.

"The largest exodus ever witnessed in the last decade and a half is ongoing in Mogadishu," independent broadcaster Shabelle said on its Web site.

"All businesses are closed and all streets are abandoned."

As the battles intensified on Friday, insurgents shot down an Ethiopian helicopter gunship with a missile. Ugandan peacekeepers pulled two dead crew members from the wreckage.

Somalia's envoy to Ethiopia told reporters the attacks were only targeting insurgent strongholds where local elders had failed to convince rebels to disarm.

Many analysts say Addis Ababa seems bent on obliterating the insurgents and their clan militia allies, who have been emboldened by recent strikes including the downing of a plane serving an African peacekeeping mission.

But the experts say it could have the opposite effect of turning Mogadishu's people further against their Christian-led neighbour or drawing in foreign Muslim jihadists.

Despite the fighting, Somalia's interim government remains confident a reconciliation meeting of elders, politicians and former warlords planned for April 16 will go ahead in the city.

The mandate for the administration, which is the 14th attempt to restore central rule in Somalia since 1991, runs out in 2009, after which, in theory, there should be elections.

The African Union (AU) has sent 1,200 Ugandan troops to help the government, but they have been attacked. Other African nations are baulking at sending more soldiers to bring the AU force to its planned strength of 8,000.

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