A Stockholm arbitration court has sided with Ukraine in three preliminary rulings in a long-running dispute between Moscow and Kyiv over natural gas.

Ukraine's Naftogaz energy company signed a 10-year gas-purchase contract with Russian gas giant Gazprom in 2009 after being cut off from Russian deliveries in the middle of the winter heating season.

Ukraine later said the contract had unfavorable terms and sought to win back $18 billion that it argues it overpaid to Gazprom in 2011-15 after being charged "nonmarket" prices.

Gazprom dismissed Ukraine's claims and demanded $37.1 billion to compensate for the volumes of gas Naftogaz allegedly failed to buy under the terms of a disputed "take-or-pay" provision of the contract.

The take-or-pay provision requires Ukraine to purchase a set amount of gas and pay for the volumes it does not buy.

But the Stockholm arbitration court on May 31 sided with Ukraine on the take-or-pay issue, Naftogaz said, and the court also agreed that Ukraine has a right to resell the gas it imports from Gazprom.

A third ruling found that Naftogaz "is entitled to a market-reflective adjustment of the price formula," the Ukrainian company said.