Among those missing are an eight-year-old girl and a 54-year-old woman, local authorities reported.
ADVERTISEMENTFlash floods have hit Italy's central Marche region, leaving at least 11 people dead and 50 injured.
An additional two people were said to be missing, according to the authorities.
Severe weather wreaked havoc in the coastal town of Ancona -- the region's capital -- and the villages surrounding it, with more than 400 millimetres of rain falling in the space of a few hours.
Hundreds of firefighters came to the rescue of individuals who were stranded and caught up in the floods, offering lifeboats and assistance. Reinforcements are also due to arrive from other Italian regions, including Lombardy, Emilia-Romagna, and Abruzzo.
Among those missing are an eight-year-old girl and her mother, Italian news agency ANSA reported.
Some of the victims also found themselves having to climb up trees or scramble onto rooftops to await rescue.
“It wasn’t a water bomb, it was a tsunami,” Riccardo Pasqualini, the mayor of Barbara, told Italian state radio of the sudden downpour Thursday evening that devastated his town.
The 112 emergency number received a spike in phone calls amidst an incident that the Civil Protection's regional councillor, Stefano Aguzzi, described as "unexpected".
"We hadn't seen [anything] alarming," he added. "There wasn't any time to intervene [in some towns], there were some people who maybe were driving or had left home without being aware of the risks."
A group of young people have also been sighted helping clean up the streets affected by the flooding.
They have been dubbed "angels of the mud" (angeli del fango) -- a term originally employed for youths who assisted the relief efforts after the Florence floods of 1966.
The tragedy unfolding in Marche comes after a pattern of freak thunderstorms and disruptive weather has been affecting the Italian peninsula for over a month, following a sweltering summer of record-breaking heat.
In Ancona, many have been left without electricity, while schools will be closed on Friday in the worst affected areas.