Spanish authorities have extended a ban on fertiliser after tonnes of dead fish washed up in a salt water lagoon.

       The regional government of Murcia said they would prohibit the use of inorganic nitrogen fertilisers at farms within 1,500 metres of the Mar Menor lagoon on the Mediterranean coast.

       The fertilisers can cause algae blooms, which deplete oxygen in surface water, killing fish. Murcia is one of Spain's largest exporters of fruits and vegetables.

       Spain's ecological transition minister Teresa Ribera visited the lagoon on Wednesday to see the "state of the lagoon's deterioration".

       She welcomed the regional government's plan to increase financial penalties on offenders. She said that of more than 400 sanctions currently imposed by authorities, some 200 are in court and more than 200 remain unresolved.

       Greenpeace Spain criticised "decades of neglect" that brought Europe's "largest saltwater lagoon to the brink of collapse".

       The environmental organisation said seagrass and animals had disappeared in the lagoon.

       The Murcia government estimates that every day around 5 metric tonnes of fertiliser runoffs from nearby farms are washed into the lagoon.