KYIV (Reuters) -- Armed men have burst into the headquarters of Ukraine's state energy firm Naftogaz, in what the Ukrainian Security Service (SBU) said was part of a criminal inquiry.
Both Naftogaz and Russia's Gazprom said the raid could threaten Ukraine's latest payment for Russian natural gas, due by the weekend. Ukraine is the main transit route for Russian gas exports to Europe, and a price dispute between the two neighbors disrupted supplies to many countries in January.
"A standard investigation is now proceeding. Why make such a fuss?" SBU spokeswoman Maryna Ostapenko said by telephone.
"This is an investigative group, not people in masks. And let no one at Naftogaz tell lies by saying they weren't told about it. Everyone knew that a group would be taking out documents.... This is all within the framework of the law."
Naftogaz was at the heart of the January dispute with Russia that led to gas cuts for millions of people in Southeast Europe before Prime Minister Yulia Tymoshenko clinched a supply deal in which she accepted a considerable rise in prices for Ukraine.
Naftogaz has pledged to pay Gazprom for February's supplies in the coming days.
The SBU is responsible to President Viktor Yushchenko, Tymoshenko's close ally in the 2004 pro-Western Orange Revolution, but now at odds with her on virtually all issues.
The president has criticized the contract clinched by Tymoshenko in January as a "defeat" for Ukraine but has made no suggestion it could be reviewed or overturned.
Tymoshenko was holding talks in Paris.
A Reuter’s witness said the men entering Naftogaz headquarters were wearing black uniforms and helmets with visors.
"At the moment, it is clear that the building is sealed off. There are other units standing in the courtyard," Naftogaz spokesman Valentyn Zemlyansky told reporters.
"It is only from news reports that we know that the SBU is conducting some sort of investigation. But how can you call this an investigation when they are carrying weapons?"
A senior Naftogaz official told reporters the security services were searching for original copies of the new gas contract -- vital to ensure payment for supplies.
Serhiy Davydenko, head of Naftogaz's legal department, said investigators were "expecting to come away with the original documents signed between Naftogaz and Gazprom in January in Moscow.
"If we don't have these contracts, then Naftogaz cannot bring the gas through customs or the transit of gas. It cannot carry out its obligations under these contracts."
The customs service earlier said it had legally taken over 11 billion cubic meters of gas owned by RosUkrEnergo -- a gas intermediary between Russia and Ukraine that had been slammed by Tymoshenko as corrupt.
The contract with Gazprom eliminated RosUkrEnergo as a supply middleman between the two countries.
Naftogaz's acting head had planned a morning meeting with President Viktor Yushchenko on March 4, but the meeting was cancelled.