Russia's Finance Ministry says Ukraine has failed to repay a $3 billion Eurobond debt owed to Russia by December 21.
"It means Ukraine has preferred a default on its debt liabilities to talks on an offer made by the Russian president…in November 2015," the ministry said on December 21.
Ukrainian Prime Minister Arseniy Yatsenyuk announced on December 18 a "moratorium" on any debt repayments to Russia. He did not indicate when Ukraine would be ready to repay the debt.
That effectively means that Ukraine is defaulting on the debt. Moscow has previously said it will take Ukraine to court if it failed to pay on time.
The Eurobond matures on December 20, but Ukraine has a 10-day grace period before it will be considered officially to be in default.
Yatsenyuk also said Ukraine would cancel payments on $507 million of Ukrainian commercial debt held by Russian banks.
Relations between the two neighbors soured after Russia annexed the Crimean peninsula in March 2014 and threw its backing behind separatists in eastern Ukraine.
Ukrainian leaders have accused Moscow of sending troops and weapons to the east, a claim the Kremlin has denied.