Naftogaz has taken a step to sue Gazprom hours after Russian state-owned concern filed a USD 4.5 B lawsuit against its Ukrainian counterpart over unpaid debts.

The Ukrainian company for its part claims it is to be given back USD 6 B for alleged "overpayment" to Gazprom, the Moscow Times has reported.

In the lawsuit at the Stockholm Arbitration Court, it has also demanded that a "fair and marker price" be set for gas deliveries from Russia.

Kiev says that since 2010 it has been overpaying for its supplies.

Monday, June 16 was the deadline for Ukraine to transfer an instalment of its gas debt to Russia.

Moscow however said it was switching to prepayment mode as it had not received the USD 1.95 B that was due until 06:00 GMT.

The two countries earlier failed to agree on gas pricing after Russia rejected Ukraine's latest offer of USD 326.5 per thousand cubic meters, sharply down from the current USD 485. 

Naftogaz later confirmed that Gazprom has restricted the flow of gas to its pipeline system, with the exception of deliveries to Europe, which are still being transported via the country's network.

The country's Energy Minister Yuriy Prodan has also urged Europe to stay calm, as gas transit will be guaranteed.

Ukraine currently has some 14 bcm of gas in its deposits and this is to suffice for some weeks.

The Kiev government seems adamant that it cannot accept the pricing regime adopted by Gazprom after it announced a 80% price hike for the gas it sells to Ukraine.

"Ukrainians would not annually draw USD 5 B out of their pockets for Russia to buy with them weapons, tanks, airplanes and bomb Ukraine's territory," UNIAN news agency quoted Prime Minister Arseniy Yatsenyuk as saying.