NAIROBI (Reuters) - The United Nations on Tuesday urged Somali troops, African Union peacekeepers and Islamist militants not to indiscriminately shell densely populated areas of the capital Mogadishu.

Thousands of civilians have been killed in fighting between the Western-backed Somali government and Islamist rebels in the last several years in the Horn of Africa nation, which has been mired in civil war since the ousting of a dictator in 1991.

The death toll from the latest bout of violence on Monday rose to at least 26, a rights group said, with scores wounded.

"These are clear violations of the law of war," Mark Bowden, the U.N. Humanitarian Coordinator for Somalia, said.

"I am deeply disturbed by the plight of civilians in Mogadishu, who are caught amidst the warring parties," he said.

Monday's fighting saw shelling by insurgents, triggering return volleys of artillery from the A.U.-backed Somali forces. A school, a crowded market, a U.N. compound as well as residential areas were hit, the United Nations said.

Al Shabaab rebels, who profess loyalty to al Qaeda, have been fighting Somalia's government since 2007 and Western powers say the anarchic nation is a breeding ground for extremism.

The U.N. World Health Organisation (WHO) said that medics in Mogadishu's hospitals were being overwhelmed by casualties.

"STRETCHED TO LIMIT"

"In March 2010 alone, at least 900 conflict-related injuries and 30 deaths were reported at Mogadishu's three main hospitals," WHO spokesman Paul Garwood told reporters in Geneva.

The United Nations estimates some 100,000 people have been displaced from Mogadishu since the beginning of the year.

Children aged under 5 accounted for 10 percent of reported injuries which included shrapnel and gunshot wounds, fractures and crush injuries, he said.

Garwood told Reuters: "Health care workers are struggling to cope, they are overwhelmed with the huge increase in wounded. It is stretching an already weak health care system to the limit."

Only 250 qualified doctors, 860 nurses and 116 midwives work today in Somalia, home to the lowest number of health workers of any country in the Horn of Africa or Middle East, WHO said.

Somalia had 300 doctors as recently as 2006, but some have fled the country, part of a "brain drain", while others have been victims of violence, including some killed by a blast at a graduation ceremony last December, Garwood said.