Russia says it's suing Ukraine for defaulting on a $3 billion Eurobond debt owed to Moscow.
Russia's Finance Ministry said in a statement on January 1 that Kyiv is "in a state of default" regarding its obligations toward Moscow and legal proceedings would ensue.
The ministry said it is filing a lawsuit against Ukraine with a British court.
Ukrainian Prime Minister Arseniy Yatsenyuk said on December 18 a "moratorium" on any debt repayments to Russia. He did not indicate when Ukraine would be ready to repay the debt, which matured in December.
The debt stems back to 2013 when Russia loaned $3 billion to the Kremlin-backed President Viktor Yanukovych, before pro-European protests that led to his downfall.
Kyiv says it is not a sovereign loan by one state to another but rather a transaction made via the financial markets that is subject to terms agreed with other creditors.
In August, Kyiv reached a restructuring deal with private creditors, including major banks, which agreed to reduce their claim by 20 percent.
Moscow insists that the loan cannot be considered private debt and has refused such conditions.
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.
Based on reporting by AFP and Interfax