Ukraine’s state energy group Naftogaz said it has asked Swiss courts to enforce a ruling that states it should receive $2.6 billion (2 billion pounds) from Russian giant Gazprom after a long-running legal battle.

“The company confirms that it has asked the Swiss courts to enforce the award, and understands that Swiss authorities already have taken measures against Gazprom’s assets there,” Naftogaz said in a statement.

The ruling, made by the Stockholm arbitration court in February, was meant to conclude a legal battle over gas deliveries. But Naftogaz said Gazprom had not complied with the ruling which obliged the Russian company to resume gas supplies to Ukraine at a market reflective price and to pay $2.6 billion, according to Reuters.

“Despite the awards and timely pre-payment by Naftogaz, Gazprom refused to resume deliveries to Naftogaz as agreed on 1 March 2018,” Naftogaz said in its statement.

Naftogaz said last month that it would go to court to seize Gazprom assets in Europe, but would not touch Russian gas transits through Ukraine.

“Gazprom will defend its right in accordance with the applicable law,” Gazprom said in a statement on Wednesday.

The legal battle has run alongside Ukraine’s broader political stand-off with Russia.

Gazprom appealed against the Stockholm ruling in April, and the case is ongoing.