Morocco navy returns 400 migrants heading to Spain
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 Organisation 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.
Libyan migrants
Meanwhile, Tunisia's defence ministry said on Monday it had prevented 53 Ivorian and Sudanese citizens from illegally entering the country, a move condemned by NGOs as a violation of rights.
On Sunday, two groups - 33 Ivorians and 20 Sudanese - arriving from Libya by land were arrested by military units in the southern border area of Medenine and sent back to Libyan territory, the defence ministry said.
The units asked the groups to enter via official border crossings, the ministry said in a statement.
But on a video shared on Sunday on social media, the apparent migrants - who include women and children - say that they have been abandoned in the desert by the Tunisian authorities without food or water.
"We don't know what to do... we need help," one of them said, describing his group as Ivorian.
The video was reposted by the Tunisian Forum for Economic and Social Rights.
In a joint statement, that NGO and other rights groups decried a "violation of migrants' rights".
The statement said 36 - rather than 33 - Ivorians had been intercepted, including 11 women and three babies, who were "abandoned in a tough climate".
The NGOs requested on Sunday that the Tunisian authorities allow the "migrants to enter urgently into Tunisian territory so that they can be taken care of by humanitarian organisations".
A July report by the Tunisian Forum for Economic and Social Rights said that the number of asylum-seekers arriving via land borders more than doubled in the first half of 2019.
A total of 1,008 people entered Tunisia illegally over six months, compared to 417 in the first half of 2018, it said.
Many were sub-Saharan Africans arriving through Libya.
"The aim of the majority of them is to leave (Tunisia) towards Europe by sea, while some have left Libya fleeing the security situation," the report said.
Follow us on Twitter: @The_NewArab