"If it doesn't rain, we'll have to start rationing water in Tehran between late November and early December", Pezeshkian said in a speech broadcast on state TV.

"Even if we do ration it and it still doesn't rain by then, we'll run out of water, we'll have to evacuate Tehran."

It was not immediately clear how the capital would be evacuated.

Iran as a whole has been affected by dwindling rainfall, but the sprawling megacity of more than 10 million people is particularly vulnerable.

On Sunday, the director of the regional water company, Behzad Parsa, warned in state media that there was just two weeks' worth of water left in the main reservoir serving Tehran.

Meanwhile, Tasnim News Agency reported that the country's precipitation level this year had reached just 152 millimetres, a 40 percent decline compared to the 57-year average.

The agency quoted Mohammad Reza Kavianpour, head of the Water Research Institute, as saying that several provinces "experienced a 50 to 80 percent decline in precipitation."

He added that storage in the capital's dams in the new water year, which began in late September, was around 250 million cubic metres, roughly half of the 490 million cubic metres recorded in the 2023-24 water year.

"We must prepare ourselves for a critical situation," he said, according to Tasnim.

Water levels in the reservoirs supplying the capital have fallen to their lowest in decades, according to Mohsen Ardakani, director general of the provincial water company covering Tehran.

"In the last six months, the population has saved 10 percent of its water consumption," Ardakani said on state TV on Wednesday.

"If this figure reaches 20 percent, we will be able to ensure a stable water situation for the next one to two months when the rains start again," he added.

Winter months in Tehran usually bring rainfall and often snow.

A sharp decline in rainfall and reduced dam inflow have deepened the crisis, as Iran faces one of its worst droughts in decades.

As a conservation measure, water supplies have reportedly been cut off in several neighbourhoods of Tehran in recent months, while outages were frequent throughout the summer.

In July and August, two public holidays were declared to save water and energy, as power cuts became an almost daily occurrence amid a severe heatwave.