Police in Mauritania say at least eight people have died while trying to illegally enter the Spanish Canary Islands by boat from Senegal.
One report said their bodies were found washed up on a beach and it was suggested scores more may have drowned.
Mauritania said it had rescued more than 100 other African migrants before their boats sank on Saturday night.
Spain is appealing for more European Union funding to curb the record numbers of migrants arriving illegally.
A Mauritanian police official who requested anonymity told the AFP news agency that migrant boats usually carried 90 to 96 people and there were probably "more bodies in the ocean".
The dead people found were certainly illegal immigrants, he added, because there were no reports of missing fishermen.
Record number
Spain's Deputy Prime Minister, Maria Teresa Fernandez de la Vega, is due to travel to Finland on Tuesday to ask the EU presidency for greater aid in stopping uncontrolled migration. More than 18,000 migrants, mostly young men, have arrived on the Canary Islands in fishing boats this year.
Nearly as many arrived in the first three weeks of August as in the whole of last year, according to official figures.
"We need more political engagement and more resources," the minister told reporters on Friday.