An explosion of jellyfish in the Mediterranean threatens both the sea's biodiversity and the health of tens of thousands of summer tourists, scientists warn.

Global warming and overfishing are being blamed for the dramatic increase in the numbers of the venomous sea creatures, they said.

"I flew along a 300-kilometer (185-mile) stretch of coastline on 21 April and saw millions of jellyfish," Stefano Piraino of Salento University in southern Italy told Britain's The Guardian newspaper.

In a Mediterranean-wide project to track the rise in the number of jellyfish, "citizen scientists" armed with smartphones and a special app have been enlisted to track the creatures along thousands of miles of Mediterranean coastline.

A surge in jellyfish numbers appears to be part of a global phenomenon, researchers said, with a rise in numbers reported from most coastal areas studied around the world.

"It is a growing problem in the Mediterranean, as it is in the rest of the world," Josep Maria Gili at the Institute of Marine Sciences in Barcelona, Spain, said.

At least 150,000 people have been treated for jellyfish stings around the Mediterranean each summer, researchers said.

Along with Spanish beaches, other badly hit coastlines include Sardinia, Sicily, Malta and the eastern Mediterranean beaches of Israel and Lebanon.