At least 45 people drowned and 37 others were rescued by fishermen off Libya's coastal town of Zwara.
A boat carrying dozens of migrants and refugees bound for Europe has capsized in the Mediterranean Sea off Libya
At least 45 people drowned or were missing and presumed dead, the UN said.
The UN agencies for refugees and migration said in a statement that 37 survivors from Monday's sinking, who were rescued by fishermen, said at least 45 others, including five children, had died when the engine of the vessel they were aboard exploded off the Libyan coastal town of Zwara (Zuwara).
The survivors, mainly from Senegal, Mali, Chad, and Ghana, were later detained when they disembarked in Libya, UNHCR and the International Organization for Migration (IOM) said of the deadliest shipwreck so far this year.
The two organisations called on countries to urgently review their approach to the tragic situation, which routinely sees migrants desperate to leave war-ravaged Libya for Europe perish at sea.
Absence of 'dedicated' rescue plan
The latest tragedy brings to 302 the number of migrants and refugees known to have perished on the route so far this year, they said, stressing that the actual figure was likely much higher.
"There is an urgent need to strengthen the current search and rescue capacity to respond to distress calls," they said, adding that "there remains a continued absence of any dedicated, EU-led search and rescue programme".
At the same time the non-governmental organisations, who have taken on much of the burden of saving lives in the Mediterranean as European states scaled back state-led efforts, were facing numerous legal and logistical restrictions, they said, urging countries to stop impeding their work.
"UNHCR and IOM are deeply concerned by recent delays in rescue and disembarkation," they said, stressing that "delays recorded in recent months, and failure to assist, are unacceptable and put lives at avoidable risk".
They also reiterated their stance that Libya, which has become a key route for irregular migration to Europe since the 2011 civil war, is not a safe destination to return refugees and migrants to.
Migrants in the country, they pointed out, are "at risk of the ongoing conflict, severe human rights violations, and arbitrary detention post-disembarkation".
Returned to Libya
The UN agencies lamented that responsibility for rescues was increasingly being left to Libyan state vessels, pointing out that this had led to more than 7,000 people being returned to Libya so far this year alone.
"Any assistance and responsibilities assigned to Libyan search and rescue entities should be made conditional on no-one being arbitrarily detained, ill-treated or subjected to human rights violations post-disembarkation," they said.
"Without such guarantees, support should be reconsidered, and search and rescue responsibilities redefined."
'Panicking and screaming'
Alarm Phone, an independent support group for migrants and refugees crossing the Mediterranean, said it received a call on Saturday from someone on a migrant vessel "panicking and screaming" that passengers were about to die.
The migrants, among them five women, two of them pregnant, said the boat engine had stopped working and they did not have any food or water, the group said.
Alarm Phone said it alerted Libyan, Maltese, Italian and Tunisian authorities and provided them with relevant details on the boat.
It was not immediately clear that this was the same vessel that capsized off Zwara.
Alarm Phone said the last time it was in contact with the boat was late Saturday.
"We urge states to swiftly respond to these incidents," the UN agencies said. "Delays recorded in recent months, and failure to assist, are unacceptable and put lives at avoidable risk."
Seeking better life in Europe
The shipwreck was the latest maritime disaster involving migrants and refugees seeking a better life in Europe. In June, a dozen people were missing and feared drowned off the coastal town of Zawiya, about 48 kilometres west of the capital, Tripoli.
Libya, which descended into chaos following the 2011 uprising that toppled and killed Muammar Gaddafi, has emerged as a major transit point for African and Arab migrants fleeing war and poverty to Europe.
Most migrants make the perilous journey in ill-equipped and unsafe rubber boats. The IOM said in March that its estimated death toll among migrants who tried to cross the Mediterranean passed the "grim milestone" of 20,000 deaths since 2014.
In recent years, the European Union has partnered with the coast guard and other Libyan forces to stop the flow of migrants.
Rights groups say those efforts have left migrants at the mercy of brutal armed groups or confined in squalid and overcrowded detention centres that lack adequate food and water.
The EU agreed earlier this year to end an anti-migrant smuggler operation involving only surveillance aircraft and instead deploy military ships to concentrate on upholding a widely flouted UN arms embargo that's considered key to winding down Libya's relentless war.