Jan 7 (Reuters) - Russian strikes late on Wednesday knocked out power supplies almost entirely in two regions of southeastern Ukraine, the energy ministry said.

"As a result of the attack, Dnipropetrovsk and Zaporizhzhia regions are almost completely without electricity," the ministry said in a statement on Telegram. "Critical infrastructure is operating on reserve power."

Russian attacks have long targeted Ukraine's energy network and have intensified in recent months.

Ukraine's Prime Minister Yulia Svyrydenko said impending snowfalls and temperatures plunging overnight to minus 20 degrees Celsius (minus 4 degrees Fahrenheit) were likely to compound disruptions to power and heating.

"Ukraine's energy system is under enemy attacks every day, and energy workers are operating in extremely difficult conditions to provide people with light and heat," Svyrydenko wrote on Telegram.

"Deteriorating weather conditions put additional strain on critical infrastructure," she said.

Public broadcaster Suspilne said power cuts had hit the city of Dnipro, where the metro had stopped running, and other parts of the region.

The head of the Dnipropetrovsk regional council told the broadcaster it was uncertain when power would be restored.

Oleksandr Vilkul, head of the military administration in Kryvyi Rih, President Volodymyr Zelenskiy's home town in the region, warned residents to brace for lengthy power cuts and said generators should be brought into use as much as possible.

Ukrainian Railways said trains in the two regions and signalling systems were being switched to reserve systems and stations were being operated with generators.

Zaporizhzhia Regional Governor Ivan Fedorov said the power cuts meant that air raid sirens were not audible. He said mobile telephone networks were working in emergency conditions and asked residents to stay off the networks.

Fedorov said hospitals in the city of Zaporizhzhia were functioning normally and water supplies would be quickly restored in affected areas.

(Reporting by Ron Popeski; Editing by Jamie Freed)