Water was also rising sharply in another Russian region – Kurgan – and in neighbouring Kazakhstan the authorities said 100,000 people had been evacuated so far, as rapidly warming temperatures melted heavy snow and ice and dislodged trapped water and mud in places.

Regional authorities called for the mass evacuation of parts of Orenburg, which is home to half a million people. They said the Ural river had reached 11 metres and 43 centimetres or just over 37 feet, up from 10.87 metres reported a day earlier, and that the situation was now perilous.

“There’s a siren going off in the city. This is not a drill. There’s a mass evacuation in progress!,” Sergei Salmin, the city’s mayor, said on the Telegram messenger app.

“The flood situation in Orenburg is extremely dangerous. Over the last 10 hours, the water level in the Ural River has risen by 40 cm and now stands at 1,143 cm. These levels are dangerous.”

He called on residents to gather their documents, medicine and essential items and to abandon their homes.

Alexei Kudinov, Orenburg’s deputy mayor, had said earlier that over 360 houses and nearly 1,000 plots of land had been flooded overnight. He said the deluge was expected to reach its peak on Friday and start subsiding in two days’ time.

The village of Kaminskoye in the Kurgan region was also being evacuated on Friday morning after the water level there rose 1.4 metres (4.59 ft) overnight, Kurgan’s regional governor Vadim Shumkov said on the Telegram messaging app.

Kaminskoye is a settlement along the Tobol river which also flows through the regional centre Kurgan, a city of 300,000 people. Shumkov said a deluge could reach Kurgan in the coming days.

“We can only hope the floodplain stretches wide and the ground absorbs as much water as possible in its way,” he said, adding that a dam was being reinforced in Kurgan.

Kurgan is home to a key part of Russia’s military-industrial complex – a giant factory that produces infantry fighting vehicles for the army which are in high demand in Ukraine where the Russian military is on the offensive in some areas.

There were no reports that the factory, Kurganmashzavod, had so far been affected.

(Reporting by Reuters; Writing by Andrew Osborn; Editing by Philippa Fletcher)