Russia's Gazprom has served Ukraine's Naftogaz with a preliminary invoice of $1.66 billion for June gas supplies.

This assumes Ukraine will take up contractual volumes of 114 million cu m of gas per day, which equates to 3.42 bcm for the month at $486.5 per thousand cu m, Gazprom spokesman Sergei Kupriyanov told reporters.

"Today, Gazprom has officially notified Naftogaz Ukrainy of the change to advance payment for gas supplies in keeping with clause 5.8 of the contract signed in 2009," Kupriyanov said.

"A proforma invoice was served, based on contractual supply volumes of 114 million cubic meters a day. Given the weekend [June 1 is a Sunday], Naftogaz has until June 2 to pay this bill and, beginning on June 3, the company will receive gas for consumers in Ukraine only within in the paid volumes," he said.

"Overdue debt is currently $3.508 billion, and nobody is any doubt that this is sufficiently high and an objective reason to invoke the relevant provision of the contract on prepayment," he said.