The drafting of a lawsuit against Russian energy company Gazprom is in the final stages, Ukrainian Justice Minister Pavlo Petronko said.

Ukraine has a Friday deadline to settle a debt the Russian energy company says is for $2 billion. No payment means no gas deliveries to Ukraine starting in June, Gazprom officials said.

The deadline coincides with a second round of trilateral energy talks between Ukrainian, Russian and European officials. Under a proposal, Ukraine would settle its debt and offer another $500 million by June 7.

Ahead of the Friday talks in Berlin, Petronko said Ukrainian energy company Naftogaz sent a pre-arbitration statement to Gazprom in a challenge to its contractual relationship.

"The drafting of the lawsuit is in the final stage," he said in an interview broadcast Thursday with RBC-Ukraine.

Gazprom said Ukraine may have to pay in advance for natural gas if a deal isn't reached.

Russia meets about a quarter of the European gas needs, though most of that runs through the Soviet-era transit network in Ukraine.

The crisis is a repeat of contractual rows in 2006 and 2009, but has escalated since political upheaval in Ukraine last year.

The World Bank said Wednesday the situation with Ukraine could slow the Russian economy.