MOSCOW, June 17 (RAPSI) – Gazprom may file another lawsuit with the Arbitration Institute of the Stockholm Chamber of Commerce to recover $18 billion from Ukraine, according to Gazprom CEO Alexei Miller.

Gazprom has already filed a lawsuit in Stockholm to recover Ukraine’s $4.5 billion debt for gas, the Russian gas supplier said in a statement on Monday.

According to Gazprom, a lawsuit was filed because Naftogaz failed to honor the provisions of the 2009 gas supply contract and owes Gazprom considerable funds for the gas delivered. Also, Gazprom says Naftogaz is not paying for current deliveries.

On Monday, Gazprom announced that it would use a prepayment system for natural gas distribution to Ukraine as of 10 AM Moscow time.  Naftogaz Ukraine is Ukraine’s largest oil and gas company. It produces over 97% of the oil and gas in the country.

Ukraine is requesting that Gazprom review a number of provisions in its contract with Naftogaz, primarily a reduction in the price, which Kiev believes is unreasonably high. Ukrainian Energy Minister Yuriy Prodan said the existing debt for Russian gas, which Gazprom estimated at $3.5 billion, could also be reduced.