Russian state-owned gas firm Gazprom says it has cut off supplies to Ukraine after negotiators failed to resolve a payment dispute before a key deadline expired.

Representatives from Ukraine, Russia and the European Union held meetings over the weekend in an effort to avert the crisis, but no agreement was reached.

Gazprom said Monday that Ukraine’s total debt is $4.5 billion. The state-owned gas firm will now only deliver gas that Ukraine has paid for in advance.

Both sides said they have filed claims with an international arbitration court in Stockholm.

The gas dispute between Moscow and Kiev has escalated as relations between the two countries have deteriorated.

Europe and the U.S. have imposed sanctions on Russia for its annexation of Crimea, while analysts have accused Russia of using natural gas supplies as a political tool.

In recent weeks, violence has again flared in eastern Ukraine as government forces clashed with pro-Russian militants. The military conflict was clearly having an effect on gas negotiations.

Europe relies on Russia for more than 30% of its gas, and half of that is pumped through Ukraine. Analysts worry that a disruption in supplies to Ukraine could hurt European companies and households.

Kupriyanov said Monday that “gas designated for European consumers is flowing in full accordance with the contract’s figures.”