The Libyan National Army's (LNA) operation against Tripoli entered its fifth day on Monday with Khalifa Haftar's forces raiding the capital's Mitiga airport.
She said: "The attack near Mitiga airport [on Monday] that left many civilians in Tripoli stranded brought into stark focus the imperative for all parties to respect worldwide humanitarian law, and to take all possible measures to protect civilians and civilian infrastructure, including schools, hospitals and prisons".
The Health Ministry of the Tripoli-based government said at least 27 people, including civilians, were killed and at least 27 wounded since the start of Hifter's offensive against the capital.
"Refugees told UNHCR that they were frightened and anxious about their safety, given ongoing fighting in the vicinity, and that they were left with minimal supplies", the statement said, adding the agency was working to ensure other detained migrants and refugees were not in harm's way.
The country is divided between two rival governments, with an elected parliament governing the country's east and the UN- and EU-backed Government of National Accord (GNA) ruling the west from Tripoli.
The United Nations said 2,800 people had been displaced by clashes and many more could flee, though some were trapped.
The death toll continues to rise in Libya, where attacks have killed more than 45 people in the last few days. "The United Nations is deeply concerned for the welfare of the civilian population in the ongoing violence and of the implications of the attack on the airport".
The U.N. mission to Libya (UNSMIL) called on Sunday for a truce for two hours in southern Tripoli to evacuate civilians and wounded, it said in a statement without giving details.
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As well as the United Nations, the European Union, United States and G7 bloc have all urged a ceasefire, a halt to Haftar's advance and return to negotiations.
Haftar casts himself as a foe of extremism but is viewed by opponents as a new dictator in the mould of Gaddafi, whose 40 years rule saw torture, disappearances and assassinations.
The media office for Haftar's army said 22 troops had been killed.
Since NATO-backed rebels ousted Gaddafi, Libya has been a transit point for hundreds of thousands of migrants trekking across the Sahara in hope of reaching Europe across the sea.
"This attack takes place as part of an escalation of violence on the ground in areas around Tripoli and Western Libya".
France established close relations with Mr Haftar under the Socialist government of former President Francois Hollande and his defence minister, Jean-Yves Le Drian.
Forces with the Tripoli government have announced an operation to defend the capital called "Volcano of Anger".
The video emerged as the US temporarily withdrew some of its forces from Libya due to the "increasingly complex and unpredictable" security situation, according to Marine Corps General Thomas Waldhauser, head of US Africa Command.