Gunmen have seized a key hospital in the north of Somalia's capital, Mogadishu, the International Committee of the Red Cross (ICRC) has confirmed.
Mogadishu's medical facilities are already stretched
An ICRC spokesman said dozens of wounded had been forced to flee the clearly marked Red Cross facility in violation of humanitarian law.
The militia are loyal to a group of secular warlords who have been battling rivals from the Islamic courts.
Some 200 people have been killed in recent fighting between the groups.
Somalia has had no effective government since the overthrow of President Siad Barre in 1991 but this year's clashes have been the worst in the capital for more than a decade.
A truce agreement between the two rival militias was broken last week.
The UN's humanitarian co-ordinator for Somalia warned the militias that their actions may be considered war crimes under international law.
"Any deliberate attempt to prevent wounded or civilians receiving assistance and protection during fighting in the city may constitute elements of future war crimes," Eric Laroche told AFP news agency.
Meanwhile, a senior American diplomat, Michael Zorick, who specialises in Somalia, has been removed from his post in Nairobi after expressing concerns about US support for the Mogadishu warlords, who say the Islamic Courts are sheltering al-Qaeda fighters.
The US merely says it will support those trying to stop "terrorists" setting up in Somalia but stresses its commitment to the country's transitional government, which functions from Baidoa, 250km (155 miles) north-west of the capital.
The ICRC's Pascal Hundt urged the gunmen of the warlords' Anti-Terrorism Alliance to leave so the medical staff could continue looking after the injured.
He told the BBC there were 120 patients in Keysaney Hospital when armed fighters arrived on Monday afternoon during a lull in fighting.
"We saw armed fighters entering into the hospital and taking some military positions on the roof of the hospital," he said.
He said 60 patients were taken home by their families. The other patients were still at the hospital with some Red Crescent staff.
"The situation remains very very tense," he said.
But an alliance commander told the agency their aim was "to protect [the hospital] from the Islamic courts militia that could prevent people from getting medical aid".
The Islamic Courts grouping has gradually been gaining the upper hand in recent fighting and civilians have been hiding in their homes or fleeing the sporadic but heavy battles.
The fighting began earlier this year when a group of warlords, who had divided Mogadishu into fiefdoms, united to form the Anti-Terrorism Alliance to tackle a growing Islamist force.
The Anti-Terrorism Alliance includes eight warlords, among them four ministers in the current government.
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