Firefighters did not identify the victim, but news reports said he was a German resident who had left his home to check on his 83-year-old neighbour in the downpour when the mudslide hit.

"Firefighters found the body of one of the two people missing in the landslide that hit a house in Cormons under the rubble," firefighters wrote on social media.

The incident occurred in the town of Brazzano di Cormons, in the province of Gorizia near Trieste, and was sparked by a massive downpour that caused the Torre river to overflow its banks and sent mud pouring through the streets.

The pounding rain "brought our community to its knees," said the town's mayor, Roberto Felcaro, on social media.

About 260 millimetres of water, or 10 inches, fell in approximately eight hours, he said, causing "serious damage and problems throughout our territory".

"There is a massive presence of law enforcement, firefighters, and civil protection agencies working tirelessly."

Earlier Monday, rescue workers managed to pull one man to safety from under destroyed homes. He was sent to hospital with a broken leg, local news reported.

Some 300 people were evacuated from the roofs of their homes amid the flooding, said Vannia Gava, an official from the region of Friuli-Venezia Giulia.

Video images taken by helicopter showed much of the area submerged by water.

Weather alerts have been issued across northern Italy due to bad weather.