The United Nations on Tuesday deplored "dire conditions" at migrant detention centres in Libya and said it was helping some of the 1,242 people rescued from sea over the past 10 days.
The number of migrants or asylum seekers in eight holding centres across the country had ballooned to 2,663 from 1,455 a month ago, the UN refugee agency said.
Libya has been in turmoil since the 2011 uprising that toppled and killed dictator Moamer Kadhafi, and two opposing governments and parliaments and armed groups are battling to control its cities and oil wealth.
The unrest has sent hordes of people to try to undertake the perilous sea journey to Europe on overloaded vessels, which often end in deaths and tragedy.
The Mediterranean's worst maritime disaster in decades took place this month when a boat that had left Libya with more than 750 people on board foundered. Just 26 men and two crew survived.
Migrants who are apprehended are brought back to detention centres and in some of them "more than 50 people are crowded into rooms designed for 25," UNHCR spokeswoman Ariane Rummery told reporters.
UNHCR said it was helping some of the nearly 1,250 people rescued off Libyan shores by the coast guard over the past 10 days.
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Among them were more than 200 people from the Horn of Africa, of whom four had suffered serious burn injuries in a gas explosion at a place where they were being held by traffickers, Rummery said.
The victims included a 20-year-old mother with extensive burns to her arms and legs and her two-year-old son whose face was badly burnt, she said.
"At the request of local authorities, UNHCR is helping to ease the dire conditions," the agency said, adding that most of the people held in the detention centres were Somalis, Eritreans, Ethiopians and Sudanese along with many from west Africa.
There is an urgent need "for more medical help, improved ventilation and sanitation as well as relief items," it added.
"Rising temperatures and mosquitoes, combined with poor ventilation could spread disease," Rummery warned, adding that UNHCR was distributing soap, underwear and clothes in eight of the country's 15 holding centres.
The UN agency said foreigners in Libya could be arrested for illegal immigration and be held for a period ranging from one week to a year.
It said there were a total of 36,000 refugees and asylum seekers registered on its rolls, with Syrians forming the bulk at 18,000.
UNHCR called for the release of "very vulnerable people, like pregnant women and also for alternatives to detention."