By Andrey Ostroukh 

MOSCOW--The Russian state-controlled natural gas monopoly Gazprom has billed Ukraine's state-owned energy firm Naftogaz $11.4 billion for not importing the full agreed amount of gas in 2013, Gazprom's Deputy Chief Executive Alexander Medvedev said Thursday.

The move adds pressure on the already battered economy and finances of Ukraine because its ballooning debt gives Moscow the right to demand an early repayment of a loan--which could theoretically cause a domino effect on about $20 billion of Ukraine sovereign and quasi-sovereign debt.

Moscow provided Ukraine with a $3 billion loan in late December when it bought its Eurobonds. The bond prospectus stated that the volume of total state debt and state-guaranteed debt should not at any time exceed 60% of Ukraine's annual nominal gross domestic product.

If Ukraine fails to meet this condition, Moscow may demand an early redemption. So far the ratio is below the 60% threshold, but Russia's finance ministry said it is monitoring the figures "closely".

The latest figures from Ukraine put state debt at 804 billion hryvnia ($70 billion), or 52,7% of the GDP. If Gazprom's demand is met, the ratio would rise above the 60% threshold.

Ukraine, which hopes to borrow heavily from the International Monetary Fund and other institutions to fix its economy, is under further pressure because a large proportion of its debt is nominated in foreign currencies, which grows as its currency, the hyrvnia, weakens and economy shrinks.

However, it's far from certain that Gazprom's demand will be met and Ukrainian officials have repeatedly indicated they would not recognize it. The claim results from a so-called "take-or-pay" contract, which makes the buyer pay for a contracted volume of gas, regardless of the actual amount purchased.

Under a 2009 contract with Gazprom, Ukraine has to buy a certain amount of gas per year, but in recent years it has bought much less than agreed. Gazprom has several times claimed that Ukraine must pay for gas contracted but not purchased--but Ukraine has refused.

It is not the first time that Gazprom has failed to make one of its buyers pay for contracted gas. In 2012, Gazprom claimed that the Czech unit of Germany's RWE did not fulfill its take-or-pay agreement. However, the European company won a landmark dispute with the Russian energy giant after a court ruled that it did not have to pay fines under this clause.

Ukraine, which is struggling through its worst financial crisis in decades, is heavily dependent on Gazprom's gas. However, Russia also relies on Ukrainian pipelines to supply nearly a third of Europe's gas.

In mid-April, President Vladimir Putin warned that Russia could eventually cut gas supplies to Ukraine if the country failed to pay its bills, and that European consumers could be affected. The European Union urged Russia to refrain from such measures.

Earlier this month Russia threatened to make Ukraine pay in advance for its gas imports, should Kiev fail to pay for the deliveries and not agree to a higher price. However, it has not yet introduced a pre-payment system. Russia has--as of April 1--also raised the cost of gas to Ukraine to $485.50 per 1,000 cubic meters from $268.50 per 1,000 cubic meters. Moscow said the price increase was because of Kiev's failure to pay its bills.

Gazprom said Ukraine's debt for delivered gas stood at $2.2 billion in early April and claimed that Kiev should reimburse more than $11 billion which the country saved as part of a discount agreement that Moscow recently scrapped.

Ukraine's Prime Minister Arseniy Yatsenyuk said the country doesn't recognize the price increase, as it goes against a deal signed late 2013, and will not pay its debt for gas delivery until the issue of the price is settled.

The prime minister, who has called Gazprom's actions "economic aggression," warned his country to prepare itself for Russia switching off gas supplies.

Naftogaz declined to comment.

James Marson contributed to the report

Write to andrey.ostroukh@wsj.com and alexander.kolyandr@wsj.com

Write to Andrey Ostroukh at andrey.ostroukh@wsj.com

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