The United Nations refugee agency (UNHCR) on Tuesday expressed deep sorrow after nearly 150 people were reported dead or missing following two migrant boat incidents off the coast of Mauritania last week as they attempted to reach Europe.

According to the agency, 144 people either died or remain unaccounted for in two separate incidents that occurred between July 14 and July 18, while 387 people were rescued.

Mauritania's coast guard said 122 migrants went missing after a boat carrying around 160 people from Gambia to Spain's Canary Islands ran out of fuel and drifted at sea for 25 days near the capital, Nouakchott.

In another incident, the coast guard said it rescued 179 people on Saturday from a boat that had departed from Senegal.

The discrepancy between the figures released by the UN refugee agency and the Mauritanian coast guard was not immediately explained.

Mauritania has emerged as a key departure point for migrants from sub-Saharan Africa hoping to reach Europe, driven by limited economic opportunities in their home countries.

The UN refugee agency described the journey to the Canary Islands as "one of the world's deadliest", citing the vast distances involved and the frequent use of overcrowded, unsafe boats by migrants attempting the crossing. (AFP)