Spain's coastguard was on Tuesday searching for three migrant boats reported lost at sea by an NGO a day after rescuing scores of people from another vessel near the Canary Islands.
A spokeswoman for Salvamento Maritimo told AFP a rescue plane had been deployed to the area but "did not find anything".
The coastguard has also requested other ships in the area to be on the lookout, she added.
During their searches on Monday, rescuers found a boat carrying 78 sub-Saharan migrants who were taken to Gran Canaria island, she said.
They had initially thought there were 86 on board.
The three missing boats are believed to have left the coast of Senegal in recent weeks, according to Spanish NGO Caminando Fronteras which helps migrant boats in distress.
"One is carrying around 200 people and the other two between 50 and 70 people," a spokesman for the NGO told AFP.
On Monday, Caminando founder Helena Maleno said the biggest boat had left the southern fishing town of Kafountine in the Casamance part of Senegal on June 27 with "many minors on board", quoting family sources who said they had lost contact with the vessel days ago.
Kafountine lies at least 1,700 kilometres (more than 1,000 miles) south of the Canaries.
