Russia's Gazprombank said on Monday it could demand the arrest of 5.7 billion cubic metres of natural gas in Ukraine's underground storage facilities, if Ostchem, owned by billionaire Dmytro Firtash, fails to repay $842.5 million in loans.
One of Ukraine's most influential oligarchs, Firtash was arrested in Vienna on March 12 at the request of U.S. authorities, which have been investigating him since 2006.
Firtash was later released on bail.
(Reporting By Alexei Anishchuk; Editing by Alexander Winning)