Rescue crews on Sunday renewed the search for about 48 migrants missing since their boat capsized near the Spanish island of El Hierro in what threatens to be the deadliest such incident in 30 years of crossings from Africa to the Canary Islands.

Nine people, one of them a child, have been confirmed as dead after their boat sank in the early hours of Saturday morning, emergency and rescue services said.

Rescuers were able to pick up 27 of 84 migrants who were trying to reach the Spanish coast.

A Reuters journalist said one coastguard vessel had left the island of El Hierro on Sunday to renew the search. More rescue craft are expected to follow, along with air support.

Spanish authorities said the migrants were from Mali, Mauritania and Senegal.

The emergency services received a call on Saturday shortly after midnight from the boat, which was located around four miles east of El Hierro. It sank during the rescue, they said.

"They had been at sea for at least two days without food and it seems there was a panic before the boat capsized," Anselmo Pestana, the Spanish government representative in the Canary Islands, told reporters on Saturday.

Wind and poor visibility made the rescue extremely difficult, he added.

Among the dead was a child aged between 12-15, according to the NGO Walking Borders, which helps migrants.

Three other boats reached the Canary Islands during the night, carrying 208 migrants.

Calm seas and gentle winds associated with late summer in the Atlantic Ocean off West Africa have prompted a renewed surge of migrants, local authorities said this month.

The route from Africa to the islands has seen a 154% surge in migrants this year, with 21,620 migrants crossing in the first seven months, data from the European Union's border agency Frontex showed.

In some 30 years of migrant crossings to the islands the deadliest shipwreck recorded to date occurred in 2009 off the island of Lanzarote when 25 people died.