Dozens feared drowned after migrant boat sinks off Mauritania
In a tweet, the agency's special envoy for the central Mediterranean, Vincent Cochetel, said one person from the West African state of Guinea had survived.
The boat sank off the northern city of Nouadhibou, Mauritania's second largest, according to Cochetel, without specifying when the incident happened.
"New shipwreck off the coast of Nouadhibou #Mauritania of approximately 40 persons on board, there is one survivor (from Guinea)" Cochetel said on Twitter.
Details about the event remain unclear, however, and accounts are conflicting.
A Mauritanian security official, who declined to be named, told AFP that the sinking did not take place in Mauritanian waters.
The official interviewed the Guinean survivor from a hospital bed in Mauritania, however, who said that he and his friends had attempted to travel from Morocco to Spain's Canary Islands.
After the boat's engines failed, the passengers aboard the vessel reportedly leapt into the open sea.
"UNHCR & IOM (International Organization for Migration) along with authorities & partners are trying to step up efforts to prevent such tragedies, but traffickers keep lying to their clients," Cochetel tweeted.
Charlie Yaxley, the UN refugee agency's spokesman for Africa, said that very little information had been confirmed, but that UN officials are "on site providing assistance".
In that incident, 83 people survived after swimming ashore when their boat hit a rock and capsized.
Attempts to reach the Canary Islands from West African countries have increased in recent years as authorities have clamped down on crossings to Europe from Libya.