The UN criticises the “sharp reduction” in efforts by European countries to carry out search and rescue operations.
Spanish authorities have found a boat near the Canary Islands with 10 dead migrants on board, while another another 45 have perished in a shipwreck off the Libyan coast.
A plane spotted the first vessel while searching for a missing boat that had left Mauritania on 15 August with about 40 people on board and was believed to be heading to the Spanish islands off north-west Africa.
Two Spanish Maritime Rescue Service vessels reached the shipwreck about 85 miles south of the island of Gran Canaria.
The journey from western Africa to the Canary Islands has increasingly attracted migrants wanting to reach Europe without crossing the Mediterranean, where authorities have cracked down on boats.
It comes as 45 migrants - mainly from African countries - were found dead in a shipwreck off the coast of Libya.
The UN said it was “deeply concerned” and criticised the “sharp reduction” in efforts by European countries to respond to distress calls and carry out search and rescue operations.
“We urge states to swiftly respond to these incidents and systematically provide a predictable port of safety to people rescued at sea,” the UN Refugee Agency said in a joint statement with the International Organization for Migration. “Delays recorded in recent months, and failure to assist, are unacceptable and put lives at avoidable risk.”
NGO boats have stepped in to save lives “in the continued absence of any dedicated, EU-led search and rescue programme” - but this is not enough, the UN said.
When commercial vessels are the nearest boat capable of carrying out a rescue, the agency said they need to be quickly given a safe location for disembarking the rescued passengers.
They “should not be instructed” to return people to Libya, where they are at risk of “severe human rights violations” and “arbitrary detention”.
Libyan authorities are increasingly taking responsibility for rescue operations, which has led to 7,000 people being returned to the country since January.
The agency welcomed the efforts of “two-thirds” of European countries who have continued to receive refugees despite the challenges posed by COVID-19.
“The pandemic should not be used as an excuse to deny people access to all forms of international protection,” the UN added.
More than 300 migrants have died trying to cross the sea from Libya to Europe so far this year.