The floods that have tossed vehicles, collapsed bridges and covered towns with mud are the European country's deadliest such disaster in decades, with almost all deaths in the eastern Valencia region.

Rescuers equipped with drones and sniffer dogs waded through water and rummaged through debris in the increasingly desperate search for the "dozens" of people the authorities believe are still missing.

The government is deploying an extra 500 troops to the stricken areas to bolster the 1,200 already on site for search, rescue and logistics tasks.

Some cut-off areas remain without water, food or power three days after the floods began, and many roads and rail lines remain inaccessible, raising fears the death toll will climb.

"There is still a pile of cars in the industrial estate, mountains and mountains of cars," Amparo Fort, mayor of the town of Chiva, told public radio RNE.

"Many must be empty, but we are sure others are occupied," Fort said.

According to national weather service AEMET, the town west of Valencia city recorded 491 mm of rain in just eight hours on Tuesday -- almost equalling a year's worth.

- 'People are desperate' -

Government minister Angel Victor Torres on Thursday vowed an uncompromising response to "looting" and "insults" to the security forces and announced 39 arrests, in a sign civil order was breaking down in some places.

In the Valencia region town of Aldaia, Fernando Lozano told AFP he saw thieves stealing trousers from an abandoned supermarket as "people are a bit desperate".

"Until this is normalised and the supermarket opens, it's going to be very bad here."

Slabs of tarmac from destroyed roads littered fields inundated with murky brown water and mud caked the ground floor of homes in the Valencia region, AFP journalists saw.

Hundreds of people spent a third night without power or communications and slept in temporary shelters. The courthouse in Valencia city has been converted into a morgue.

AEMET placed part of the southern Andalusia region on the highest alert level for torrential rain on Friday and maintained warnings for Valencia and Catalonia on the eastern coast.

The powerful storm that sparked the flood formed as cold air moved over the warm waters of the Mediterranean and is common for the time of year.

But scientists say climate change driven by human activity is increasing the ferocity, length and frequency of such extreme weather events.