A UN Support Mission in Libya (UNSMIL) statement said Bernardino Leon "strongly condemned the renewed outbreak of violence in different neighbourhoods of the Libyan capital that caused many casualties and endangered the lives of civilians".
It said "initial reports indicate that three civilians were killed, including a young girl", since fighting broke out Friday in Fashloum, a central district of Tripoli.
"There have also been reports of abductions of civilians and burning of houses," the statement added yesterday.
He called on the sides to "exert all efforts to bring an immediate end to the fighting in Tripoli and elsewhere, and to undertake all necessary measures to create a more conducive environment for the on-going dialogue".
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And he urged them to ensure that civilians and their property "are not targeted in the fighting and that international human rights and humanitarian law are respected.
"All civilians abducted should be released immediately, and the wounded given access to medical treatment," Leon added.
On March 24, UNSMIL unveiled a six-point plan to end the crisis, including the formation of a transitional unity government until a new constitution is adopted and elections held.
Talks on that plan are under way in Morocco, where another round might take place today.