Russia has pledged to supply electricity to separatist-controlled areas in eastern Ukraine after the Kiev-based Ukrainian government cut off power due to unpaid bills.

Boris Gryzlov, the Russian envoy mediating talks between Russia-backed separatists and the Ukrainian government, said on Tuesday that the Russian government has decided to help provide Luhansk region with electricity.

Dmitry Peskov, Kremlin spokesperson, defended the decision as a humanitarian mission, but would not disclose how the power supply would be organised in the Ukrainian region that borders Russia.

He also said that the move was "another step by Ukraine on the road to rejecting territory" which has been under rebel control since 2014.

INTERACTIVE: Ukraine divided - stories from warring sides

Ukraine has announced the decision to stop supplying power to Luhansk because it accumulated more than $97m in unpaid electricity charges, and local news media reported the supply was cut off close to midnight on Monday.

"This night, the power supply to the temporarily occupied territory of Luhansk region was completely halted," Vsevolod Kovalchuk, head of the state power distributor Ukrenergo, said on Facebook.

Sergei Ivanushkin, Luhansk's self-proclaimed emergencies minister, said the power outage in the region's biggest cities lasted for less than 40 minutes because "we switched to our own resources".

Vladislav Deinego , a regional separatist leader, later told Russia's Interfax news agency that Russia sent electricity through existing power lines.

More than 10,000 dead

Ukraine cut gas supplies to Luhansk in 2015, also blaming unpaid debts, and imposed a trade blockade on the occupied regions in March.

Three years after Russia annexed the Crimea Peninsula, tensions between Ukraine and separatists in the Russian-held eastern part of the country remain high and a 2015 ceasefire is violated regularly.

The conflict has claimed more than 10,000 lives.

On Sunday, an American paramedic working for European security watchdog OSCE's monitoring mission in eastern Ukraine was killed and two others injured when their vehicle struck a landmine near a village of Pryshyb, close to Luhansk.