Russia is using its energy supplies "as a tool of coercion" against Ukraine, State Department spokeswoman Jen Psaki said, as fears mounted that tensions between the countries could trigger natural gas shortages throughout Europe.
Also, NATO released satellite images showing Russian troops and heavy equipment near the Ukranian border.
Psaki said Thursday that Moscow is charging Ukraine prices for natural gas that are not set by the market, BBC News
reported, and the United States is working to help the country find enough gas and providing financing.
Russian President Vladimir Putin Thursday wrote to 18 European countries to warn them
that Ukraine's failure to pay off its energy debts is creating a "critical situation," and if the bill isn't paid, the supplies will be shut off.
Russian energy giant Gazprom claims Ukraine owes it $2.2 billion, recently doubling the price it must pay. Putin said Thursday that Gazprom plans to switch over to advance payments and if the obligation is not met, the company will "completely or partially cease gas deliveries."
Such gas disputes in the past have led to shortages in several European Union countries. EU officials say they have extra supplies to deal with such disruptions. Europe gets about 13 percent of its gas supply from Russia and has been working to lower its dependence on that country's supplies.
Ukraine Energy Minister Yuri Prodan said Friday that his country hopes to buy gas from Europe to shore up its supplies, Reuters reported
, as it fears Russia will cut its supplies over his country's refusal to pay Russia's "political, uneconomic price" for fuel.
Meanwhile, he told the parliament that the EU will stand behind Ukraine if Russia reduces or cuts its supplies and will make sure Moscow won't be able to send flow through alternate pipelines and bypass Ukraine.
Putin's energy warnings came as NATO defended the accuracy of satellite images of Russian troops amassed on the Ukrainian border, which Russia says actually depict a military exercise from last August.
In the images, released on Thursday, NATO said Russian deployments of 40,000 troops are shown near the Ukraine border, along with tanks, artillery, aircraft, and armored vehicles.
NATO Secretary General Anders Fogh Rasmussen said in a visit Friday to Bulgaria, a NATO member, that Russia must withdraw the troops and enter into dialogue with the West, Reuters reported. The NATO alliance is not discussing military action over Ukraine, said Rasmussen, but it is taking necessary steps to protect its partner countries.
NATO on Thursday released more images from 2013 and early 2014 that it says show the same areas on the Ukraine border were not occupied before March, and said Russia's claims are "categorically false" after Russia claimed that the satellite images were old ones, the BBC reported.
The satellite photos were taken by a commercial satellite imaging company, DigitalGlobe, The New York Times
reported, and were released during a news conference in Belgium by Gary Deakin, director of NATO's Comprehensive Crisis and Operations Management Center.
Officials said they used commercial photos, not military satellite images, because classified images are difficult to declassify. DigitalGlobe, based in Colorado, has a contract agreement with the U.S. government which allows the company's images to be shared with federal agencies and allies of the United States.
Deakin said the photos, taken between March 22 and April 2, show Russia has "an array of capabilities, including aircraft, helicopters, special forces, tanks, artillery, infantry fighting vehicles. These could move in a matter of hours."
One of the images shows more than 20 helicopters in a Russian city, Belgorod, located about 25 miles from the Ukrainian border, while other images were further away.
Russian Foreign Minister Sergey Lavrov told the Interfax news service that stationing NATO troops near Russia's border will violate international agreements to "satisfy absolutely groundless fears, phobias, and ambitions of a minority of its country members."
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