Russian President Vladimir Putin has sent an appeal to a number of foreign partners, in which he informed them that Russia will switch to solely pre-paid gas supplies for Ukraine on June 1.

In early April Russia addressed European partners "with the proposal to hold immediate consultations in order to develop coordinated actions to stabilize the Ukrainian economy and to ensure stable supplies and transit of the Russian gas in accordance with the contract conditions," Putin said in his letter.

"Unfortunately, we are forced to conclude that no certain proposals on stabilizing the situation with the Ukrainian buyers fulfilling certain obligations and ensuring reliable transit were received from our partners," the document said.

Moreover, during this time the situation with payments for the Russian gas just got worse, Putin said. "Gazprom has not received a single payment for the gas supplied to Ukraine and the total debt of Ukraine grew from $2.237 billion to $3.508 billion," Putin said.

"Amid such conditions and in strict accordance with the contract, Russia has issued an invoice for pre-paid gas supplies to Ukraine and, starting from June 1, will supply gas in volumes paid for by Ukraine in advance," the letter said.