The frigate Canarias carried out the rescue in complete darkness early on Saturday off the coast of Libya as part of a joint EU mission.

The 651 migrants, who were trying to cross from North Africa to Europe, came from several sub-Saharan countries, said Spain's defence ministry.

Canarias has saved 1,958 migrants in less than three months patrolling the Mediterranean.

Hundreds of thousands of migrants and refugees have crossed the Mediterranean in smugglers' boats in hopes of a better life in Europe.

However, thousands die in the attempt each year due to treacherous waters and Libya is one of their main launching points.