Some of those taken ashore were "in a state of poor health" and received first aid aboard Moroccan coastguard vessels, before being taken to ports in the north of the country, the source said.

The majority of the 424 people -- who returned to Morocco overnight into Monday -- were from sub-Saharan Africa.

Sixteen children and 53 women were among the migrants, who were in several boats.

The number of migrants reaching Spain by sea has dropped by around 30 percent, the United Nations' migration agency says.

In the first half of this year, 10,475 reached the country, compared to 15,075 during the same period in 2018, according to figures from the International Organization for Migration.

More than 200 people died during the journey or were reported missing at sea in the first six months of 2019.

Aiming to halt the arrival of migrants, the European Union last year gave Morocco 140 million euros ($155 million) to support efforts to curb people smugglers and irregular migration.

At the end of July, Spanish Prime Minister Pedro Sanchez praised the "efforts of the Moroccan authorities" regarding migration, in a column in El Pais newspaper.

Morocco says it stopped 89,000 "attempts at illegal immigration" last year, 29,000 of which were people trying to make a sea crossing.

Rights organisations in Morocco, however, frequently decry mass arrests of migrants and their forced displacement to the south of the country.