Cars were left pilling up in a street in the southern city of Shiraz - AFP

Major floods across much of Iran have left 19 people dead and more than 90 injured, blocking roads and triggering landslides with warnings of more heavy rain to come, emergency services said Monday.

Such a widespread flood threat is unprecedented in arid Iran, which until 2018 was dealing with decades of drought.

Seventeen people were killed and 94 injured in the southern city of Shiraz, one person died in the western province of Kermanshah and another in Lorestan, also in the west, rescue services said.

The national emergency has struck in the middle of Iranian New Year holidays, with many relief workers also on vacation.

Many of those killed in Shiraz were holidaymakers caught in the flood as they entered the city in their cars.

With 25 of Iran's 31 provinces experiencing floods or facing imminent threat, the country's National Crisis Management Committee was activated at cabinet level.

View photos The streets were flooded in the northern Iranian village of Agh Ghaleh Credit: ALI DEHGHAN/AFP/Getty Images More

"I have ordered all governor-generals, all provincial managers and officials nationwide to stay at their posts throughout the next 72 hours which is the peak of the flood threat," first vice president Eshagh Jahangiri said on state television after the committee's first meeting.

Hassan Rouhani, the Iranian president, issued a statement expressing condolences to victims' families and thanking emergency services and the military for their responses to the crisis.

He demanded an "immediate investigation" into the causes of the high death toll in Shiraz.

Since the flooding, Mr Rouhani had previously been absent from public view, with his deputies and ministers taking his place in visiting disaster-struck areas.

Reports on the ultra-conservative Fars news agency that Mr Rouhani has been vacationing on the southern Gulf island of Qeshm triggered criticism from his hardline opponents and even some reformist allies.