MOSCOW, June 16 - Gazprom will now consider suing Naftogaz Ukrainy for $18 billion under take-or-pay gas contracts for 2012 and 2013.

"We could have filed [a claim] yesterday or the day before, and now we'll see," Gazprom CEO Alexei Miller told a press conference at the Interfax head office.

"This morning, right after the introduction of an advance payment system, we immediately filed a suit with Stockholm arbitration concerning nonpayment of gas for $4.5 billion. We are not ruling out that other lawsuits could be announced in the future as well," he said.