Spain's coastguard was on Tuesday looking 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 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.
The other two boats with around 120 people on board had left the Senegalese coast on June 23.
A Spanish rescue plane had initially thought the boat spotted on Monday was the one carrying some 200 people, but it was only on reaching the vessel that they realised their mistake.
"Every minute counts if we're to find these more than 300 people alive who are in three Senegalese pirogues which have disappeared in the Atlantic," Maleno tweeted on Tuesday, referring to a long wooden canoe-like vessel.
Senegal is one of the main departure points for migrants heading to Europe.
In a tweet, Senegal's main opposition leader Ousmane Sonko on Tuesday blamed the flight of migrants on the "failure of the public policies of the regime of President Macky Sall", calling it a "macabre and sad phenomenon".
Sonko, a firebrand politician and Sall's fiercest opponent, was sentenced last month to two years in prison for morally corrupting a young woman, a conviction that renders him ineligible to run in February's elections in Senegal.
The ruling led to the most serious unrest Senegal has seen in years, leaving 16 people dead, according to authorities, or 30 dead according to the opposition.
The Atlantic route to the Canaries is particularly dangerous due to strong currents, with migrants travelling in overloaded often unseaworthy boats, without enough drinking water.
Atlantic crossings surged from late 2019 after increased patrols along Europe's southern coast dramatically reduced Mediterranean crossings.
In the first six months of 2023, 7,213 migrants reached the Canary Islands by boat, interior ministry figures show.
