The Obama administration has condemned what it says are “renewed atrocities” by Libyan forces loyal to Colonel Muammar Gaddafi.
“Snipers have targeted civilians seeking medical assistance,” Secretary of State Hillary Clinton said.
Ms Clinton also said Libyan mercenaries had begun firing mortar rounds into residential areas.
She added that coalition troops would work to stop all attacks on civilians, under Nato’s command.
The Libyan regime had reportedly destroyed crucial food supply warehouses and cut off water and power to the rebel-held city of Misrata in an attempt to starve civilians into submission, Ms Clinton said in a statement on Wednesday.
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