Rescuers said they found four bodies on a migrant boat off the Canary Islands Sunday, just hours after a lone survivor had been pulled to safety, with an NGO saying it had set sail with 34 people aboard.

Spain's Salvamento Maritimo coastguard said on Twitter that it had "recovered the bodies of four people" from a boat some 150 nautical miles southwest of Gran Canaria island, with the lone survivor rescued on Saturday night by a passing merchant ship and then evacuated by helicopter.

Helena Maleno, head of Caminando Fronteras, a Spanish NGO that helps migrant boats in trouble at sea and families searching for loved ones, said on Twitter they had received a distress call from the same vessel a week earlier after it set sail from Western Sahara.

At the time there were 34 people on board.

"On Saturday 24 September we received an alert about a boat which had left the coast just south of Laayoun at dawn that morning," she tweeted, saying they had alerted rescuers.

After a week with no news, a boat was spotted south of the Canary Islands carrying "an exhausted 26-year-old man and four bodies.. By the end of the night, we knew it was the same boat that had gone missing with 34 people on board," she said.

"A new tragedy on one of the most deadly migrant routes -- the Canaries route," Maleno said, counting "33 more victims".

"Only four bodies have been found.. the rest of the victims, a total of 29, have been swallowed up by the ocean. Their families will suffer grief terribly with no bodies to be able to say goodbye to."

Since the start of the year, a total of 11,522 migrants have managed to cross the notoriously dangerous Atlantic route to Spain's Canary Islands, a figure slightly higher than the same period last year, interior ministry figures to September 15 show.

There are no official figures for those who have died en route but figures from Caminando Fronteras show that in the first six months of the year, 978 people died while trying to reach Spain, of which 938 drowned or went missing at sea.

At its shortest, the route from the Moroccan coast is around 100 kilometres (60 miles), but migrants often come from much further afield, with the distance from Mauritania more than 1,000 kilometres as the crow flies.

The Atlantic route is extremely perilous because of its strong currents, with migrants often setting sail in overcrowded ramshackle boats which are far from seaworthy.

Migrant crossings to the Atlantic archipelago began surging in late 2019 after increased patrols along Europe's southern coast dramatically reduced Mediterranean crossings.