Spain's maritime rescue service says it has rescued some 600 migrants trying to cross the Mediterranean Sea from Morocco in the past 24 hours, making it one of its busiest days so far this year.

The service said it rescued five migrants early Thursday in the Strait of Gibraltar.

On Wednesday, its vessels rescued 593 migrants — 424 in the Strait and 169 further east in the Mediterranean.

More than 9,000 migrants have reached Spain by sea so far this year, more than double the number from the same period last year.

Tens of thousands of migrants from Africa attempt the perilous sea crossing to Europe in smugglers' boats each year, with many drowning along the way.