At least 45 migrants and refugees perished off Libya this week in the deadliest shipwreck there so far this year, the United Nations said on Wednesday.

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.

The survivors, mainly from Senegal, Mali, Chad and Ghana, were later detained when they disembarked in Libya, UNHCR and the International Organisation for Migration (IOM) said.

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.

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 overthrow and killing of dictator Moamer Kadhafi in a NATO-backed uprising, is not a safe destination to return 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".

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."