Spain mourned at least 158 deaths on Friday as authorities told people in flood-stricken regions to stay at home.

An exceptionally powerful Mediterranean storm from Tuesday unleashed heavy rains and torrents of mud-filled water that swept away people and wrecked homes, with the eastern Valencia region hit hardest.

The body coordinating rescue work in Valencia announced 155 bodies had been recovered there by Thursday afternoon.

Officials in Castilla-La Mancha and Andalusia had announced a combined three deaths in their regions on Wednesday.

But "dozens and dozens" of people remain missing two days after the start of the catastrophe, government minister Angel Victor Torres told reporters, raising fears the death toll could rise further.

Some rural areas also remain inaccessible to rescuers.

"Please, stay at home... follow the calls of the emergency services," pleaded Prime Minister Pedro Sanchez.

European officials pointed on Thursday to the floods in Spain as a reminder of the self-harming effects of humans' destruction of nature, urging delegates at a deadlocked UN biodiversity conference in Colombia to "act."

European Commission envoy Florika Fink-Hooijer said the "catastrophe" in the Valencia region highlighted the link between biodiversity loss and human-caused climate change.

Worsening droughts and flooding cause the loss of plant species such as trees -- which serve as a bulwark against some of the worst effects of global warming.

"If we act on biodiversity, we at least can buffer some of the climate impacts," Fink-Hooijer said at a press conference in the city of Cali, host of the 16th Conference of Parties (COP16) to the UN's Convention on Biodiversity.

"At this COP we really have a chance to act," said the envoy, who is also the European Commission director-general for environment. (AFP)