Russian Prime Minister Dmitry Medvedev has highlighted to President Vladimir Putin, in a meeting, that Ukraine owes Russia $16bn in gas bills and loans.
According to local news agencies, cited by Reuters, Russian officials claim that Ukraine owes the country $11bn (£6.7bn, €8bn) in relation to a gas deal and a further $2bn to energy giant Gazprom in backdated bills.
Medvedev said Ukraine should pay back the alleged debt because the Kharkiv agreement, under which Russia provided cheap gas in return for the lease of the Sevastopol naval base in Crimea, would cease to be valid.
Since the naval base in Crimea is "subject to denunciation", this gives Russia a legal right to sue the Ukraine for the debt, said Medvedev.
On top of that, Russia says the embattled country also owes is $3bn following a Russian Eurobond purchase.