Russian President Vladimir Putin has written to European countries warning that Ukraine’s debt crisis has reached a “critical” stage, and could have consequences for gas transit to Europe.

Russian President Vladimir Putin has written to European countries warning that Ukraine’s debt crisis has reached a “critical” stage, and could have consequences for gas transit to Europe.

"After yesterday's meeting the president held with the government presidium, Putin sent a letter to the heads of European countries buying Russian gas," a source told ITAR-TASS on Thursday, which was shortly confirmed by the President's spokesperson Dmitry Peskov.

“The letter expressed extreme concern over the escalating situation, noting its critical nature and putting forth a number of concrete proposals, ....aimed at taking emergency measures, as the situation is urgent,” Peskov said.

The message has been delivered to several key heads of European states that buy Russian gas, according to Peskov.

Russia’s biggest gas client is Germany which imports over 25 billion cubic meters of gas from Gazprom each year, about a third of its energy needs. Italy is another big importer, also relying heavily on Russian imports as deliveries from North Africa have proved unreliable.

Bulgaria could lose the most, as it imports nearly 90 percent of natural gas supplies from Russia, almost all of which pass through Ukraine. Lithuania, Estonia, Finland, and Latvia import 100 percent of their natural gas from Russia, but not through Ukraine.

On Wednesday the President discussed Ukraine’s massive debts with leading Russian officials, asking them to refrain from any critical measures against Kiev in terms of gas supplies.

Ukraine’s total debt to Russia, including the $2.2 billion bill for gas, now stands at $16.6 billion, Prime Minister Dmitry Medvedev said on Wednesday.

The country's total debt includes unpaid bills from November and December 2013, as well as debt for February and March 2014, according to Gazprom.

Gazprom has revoked all discounts and now charges $485 per 1,000 cubic meters of gas, a price Ukraine says it will not be able to pa because it threatens Ukraine's ability to continue normal gas transit operations to Europe.

Moscow cut off gas transit through Ukraine to Europe in the winters of 2006 and 2009 over similar unpaid bills to Gazprom, which left parts of Europe without heat. Moscow claims Ukraine illegally siphoned off supplies intended for Europe during this time, an accusation Kiev denies.