Ukraine's state energy firm Naftogaz said Russia's Gazprom (GAZP.MM) is violating the terms of an EU-backed winter gas supply agreement, ahead of talks on Monday in Brussels on the issue.

"In February, the agreement was violated by the Russian side. Gas for which we had paid in advance was not delivered in full last month. The deal is continuing to be violated this month," Naftogaz on Monday quoted its chief executive Andriy Kobolev as saying.

"This violation needs to be balanced by an increase in gas deliveries to Ukraine from the EU," he said.

The European Commission invited the Russian and Ukrainian energy ministers to talks in Brussels on Monday and the two sides have confirmed that they would attend.

Naftogaz has said the implementation of a winter agreement and a preliminary discussion of a summer deal would be the main topic of the talks.

Moscow cut off supplies to Kiev last June and restored them in December in a dispute over pricing and unpaid bills that marked the third such clash in a decade.

According to the winter agreement, Kiev has to pay in advance to receive gas from Russia. Gazprom last week warned that Ukraine needed to prepay for future deliveries in March or it would stop supplies.

Naftogaz said on Friday it sent a small prepayment of $15 million to Russia "to calm the nerves of our European colleagues" who fear disruption in Russian gas transit shipments to Europe via Ukraine.