A joint Spanish and Equatorial Guinea naval operation has rescued 20 crew members on a merchant ship from a hijacking by pirates in the Gulf of Guinea, the Spanish defence ministry said Tuesday.

The captain of the Maltese-flagged vessel sounded the alarm on Sunday after their ship was approached by armed pirates, the ministry said, with the crew taking refuge in an emergency compartment.

"Once on board, the pirates used a megaphone to demand the crew surrender and give them all the money on board," the ministry added.

When the captain refused to comply, the pirates fired into the emergency compartment, the ministry said. No one was injured by gunfire.

The Spanish "Serviola" patrol ship, which carries out security and surveillance tasks in the region, received reports of the hijacking and conducted a rescue operation with an Equatorial Guinea navy vessel.

Ten pirates were arrested, according to the Equatorial Guinea authorities.

"I congratulate the heroic action of our armed forces," said Vice President of Defence and Security in Equatorial Guinea, Teodorin Nguema Obiang, in a statement read out on state radio on Tuesday.

He said the navy intervened after "a call for help from a boat... attacked by a speedboat with ten pirates on board". 

"Thanks to the swift intervention of our armed forces, (we) managed to save the crew on board and arrest ten pirates,"  he said, adding that their presumed nationality was Nigerian.

The Gulf of Guinea, a coastal zone stretching thousands of kilometres off West Africa, has now eclipsed the Gulf of Aden -- off the Horn of Africa -- as the continent's piracy hotspot.

Reports of attacks in waters between the Ivory Coast and the Democratic Republic of Congo more than doubled in 2018 compared to the year before, according to a piracy report by the International Maritime Bureau in January.

The last quarter of 2018 saw a significant spike in violence off West Africa, with vessels boarded by pirates well outside territorial waters, and crew kidnapped and taken to Nigeria where they were held for ransom, the report said.

Piracy in the Gulf, home to Sub-Saharan Africa's two main oil producers Nigeria and Angola, has seriously disrupted international shipping routes and cost the global economy billions of dollars.

Countries in the region, whose surveillance and maritime defence capabilities are limited, have been trying for several years to bolster their means of intervention and to put in place closer collaboration, with the help of Western powers.