The headquarters of Ukraine's natural gas company Naftogaz has been raided by national service personnel.
Authorities have begun questioning employees and searching for documents as part of a criminal investigation, a spokesperson for the security service of Ukraine (SBU) said.
Initial reports suggested the men who raided the company were armed and masked but the claims have been rejected.
SBU spokeswoman Maryna Ostapenko said: "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 hit the headlines earlier this year after becoming involved in a dispute with Russian state-controlled energy company Gazprom.
Disagreements over gas prices and allegations of stealing supplies led to gas flowing through Ukraine being cut-off for two weeks, affecting numerous countries across Europe.
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