Spain and Morocco are embroiled in a slowly-building diplomatic row as thousands of Moroccans exploit relaxed border controls back home to swim or paddle in inflatable boats into Europe.

By Tuesday morning, about 6,000 people had crossed the border into the Spanish city of Ceuta since the first arrivals began early Monday, the Spanish government said. The arrivals included 1,500 teenagers.

Ceuta, a city of 85,000 people, lies in North Africa on the Mediterranean Sea, separated from Morocco by a double-wide, 10-metre fence.

The sudden influx of migrants has worsened the diplomatic row between Rabat and Madrid following the Spanish government’s decision to welcome Brahim Ghali, the head of the Polisario Front, which has been fighting a guerrilla war against Morocco since its annexation of the Western Sahara in 1975.  Spain says it allowed in Mr. Ghali on health grounds, but Morocco insists the move by Madrid was intended as an affront to Rabat.

the chief of a militant group that fights for the independence of Western Sahara. Morocco annexed the sprawling nation on the west coast of Africa in 1975.

On Tuesday, migrants soaked with seawater still kept reaching Ceuta although in smaller numbers than the day before due to heightened vigilance on the Spanish side of the border, where additional police and military were deployed.

Officially, the Spanish authorities are denying the notion that Morocco is punishing Spain over the decision to grant “compassionate assistance” to Mr. Ghali.

Interior Minister Fernando Grande Marlaska only said on Tuesday that Madrid had processed the return of 1,600 migrants by Tuesday morning and that the rest would follow soon. He said that was in keeping with an agreement signed between the two countries three decades ago to return all those who swim into the territory.

A recent report by the Associated Press said many African migrants regard Ceuta and nearby Melilla, also a Spanish territory, as a gateway into Europe.

In 2020, 2,228 crossed into the two enclaves by sea or by land, often risking injuries or death. The year before the figure peaked at 7,899, according to Spain’s Interior Ministry.

On Tuesday, another eighty migrants from Africa also crossed into Melilla, 350 kilometres east of Ceuta on the North African coast.