"This is an attack on Spanish borders and on the borders of the European Union. It is unacceptable under international law," Margarita Robles said during an interview with Spanish public radio.
The minister went on to claim that Rabat was "using" minors in efforts to pressure Madrid, now regarded by the Moroccan authorities as favourable to the cause of independence for Western Sahara.
She said children as young as 7 or 8 were being allowed through by Moroccan border patrols. The Spanish police have released a photograph, apparently showing a baby being taken from the water by a member of the coast guard.
Earlier this week, an unprecedented 8,000 migrants crossed into the Spanish territory in just 36 hours.
On Tuesday, Morocco recalled its ambassador from Spain for consultation.
Spain deployed an extra 200 police officers to patrol the border.
Diplomatic tensions over Western Sahara
The sudden influx of migrants comes against a background of heightened tensions between Madrid and Rabat over Spain’s decision to allow Western Sahara independence leader, Brahim Ghali, to be treated for Covid-19 in Spain.
The arrival of Ghali in Spain last month sparked a protest from Rabat, with Morocco’s foreign ministry describing Spain’s move as “inconsistent with the spirit of partnership and good neighbourliness” and warning it would have consequences.
Morocco annexed the Western Sahara region on the west coast of Africa in 1975.
Ghali's Polisario Front is fighting for the independence of Western Sahara, a former Spanish colony now under Moroccan control.
Morocco's minister of state for human rights, Mustapha Ramid, said on Wednesday it was "clear" Spain had favoured its ties with the Polisario and Algeria over those with Morocco by hosting Ghali.
Spanish officials said the independence leader had been allowed to access medical treatment in Spain for “strictly humanitarian reasons”.
'A crisis for Spain, and for Europe'
The Spanish prime minister, Pedro Sánchez, has said anyone who entered Ceuta and the nearby Melilla enclave irregularly would be “immediately returned”, under the terms of a bilateral agreement that allows for the return of Moroccans who swim into the territory.
“This sudden arrival of irregular migrants is a serious crisis for Spain and for Europe,” he added.
The EU commissioner for home affairs, Ylva Johansson, described the situation as worrying and said: “Spanish borders are European borders.”
Around 5,000 people crossed the border from Morocco into the Spanish enclave of Ceuta on Monday. EU Home Affairs Commissioner Ylva Johansson called the migrant arrivals "unprecedented" and "worrying."https://t.co/yUO17MjvGQ
— DW Europe (@dw_europe) May 18, 2021
She added: “The most important thing now is that Morocco continues to commit to preventing irregular departures, and that those that do not have the right to stay are returned.”
She called on Morocco to do more to protect relations with the EU. “The European Union wants to build a relationship with Morocco based on trust and shared commitments. Migration is a key element in this,” she said.
Spain’s interior ministry said 5,600 people had already been sent back to Morocco. There were no new attempted entries on Wednesday.
