MOSCOW, May 15 -- Russian President Vladimir Putin called on European leaders Thursday to take concrete and active measures to help solve the Ukrainian gas debt problem and stabilize that country's shrinking economy.

"We hope that the European Commission will more actively engage in the dialogue in order to work out specific and fair solutions that will help stabilize the Ukrainian economy," Putin said in a letter, the second time since April 10, to 18 European leaders regarding the supply and transit of Russian gas across Ukraine.

He said Russia was "forced" to switch to prepayment regime as Ukraine had failed to fulfill its contractual obligations and the gas debt had reached over 3.5 billion U.S. dollars.

"(Russia's state-run gas monopoly) Gazprom has not received a single payment for the gas supplied to Ukraine ... despite the fact that Ukraine has received the first tranche of the International Monetary Fund's loan to the amount of 3.2 billion dollars," the Itar-Tass news agency quoted Putin as saying.

Noting that Russia has not received any specific proposals from European partners on how to stabilize the situation with the Ukrainian buying company, Putin said Moscow "remains open to continuation of consultations and joint actions."

In 2010, Russia and Ukraine signed a gas-for-fleet agreement, under which Moscow offered a discount for gas in exchange for extending its lease of the Black Sea fleet in Crimea for another 25 years.

In April, Moscow declared that pact no longer valid as Crimea joined Russia in March, though the West and Kiev refuse to recognize it. Following that announcement, Gazprom canceled discounts on gas supplies to Ukraine and set a price tag of 485 dollars per thousand cubic meters, which Kiev has called overpriced and temporarily suspended gas intake.

Earlier the week, Gazprom informed its Ukrainian counterpart Naftogaz that supplies of gas will be conducted under prepay option starting June. Under that scheme, Naftogaz must prepay to the Russian supplier 1.659 billion dollars before June 2 for 3.42 billion cubic meters of gas.