(Reuters) - More than 300 homes were flooded in the Russian city of Orenburg after the Ural River rose half a metre beyond its bursting point, officials said on Wednesday, as swiftly melting snow from the Ural Mountains triggered the worst flooding in decades.

The deluge of meltwater overwhelmed swathes of the Ural Mountains, western Siberia and areas of Kazakhstan close to rivers such as the Ural and Tobol, prompting the order to evacuate more than 100,000 people by late Wednesday.

Authorities said the situation was dangerous at Orenburg, where water levels in the Ural River, Europe's third-longest, had risen 50 centimetres (20 inches) by early Wednesday, just hours after hitting the critical level of 9.3 metres (30 feet) late on Tuesday.

"The night was restless," RIA state news agency citied Orenburg's First Deputy Mayor Alexei Kudinov as saying.

Hydrologists say the worse is yet to come for Orenburg, a city of around 550,000 people. The waters of the Ural River, which flows through Russia and Kazakhstan into the Caspian, may rise a further 70 cm (28 inches) by Thursday.

The Ural River burst through a dam embankment in the city of Orsk in Orenburg region, of which Orenburg is the administrative centre, at the weekend.

In Kurgan, a city on the Tobol River in the south of the Urals, sirens warned people to evacuate immediately. Regional officials said floodwaters would continue to rise for three days and predicted a "difficult situation" until the end of April.

On Wednesday, local authorities said they had closed traffic on several roads in the region to quickly deliver soil to strengthen a dam there as the flood forecast worsened and water levels in the Tobol River quickly rose 23 centimetres (9 inches).

(Reporting by Lidia Kelly in Lisbon; Editing by Muralikumar Anantharaman and Lincoln Feast.)