A group of around 20 masked and camouflaged agents from the Ukrainian security service on Wednesday raided the headquarters of the state gas company Naftogaz, the company said.
"They are outside the office of Naftogaz deputy chief Igor Didenko," said company spokesman Valentin Zemlyansky.
Naftogaz was earlier this year involved in a prolonged dispute with Russian state-controlled energy giant OAO Gazprom (GAZP.RS) which led to over a dozen European countries being deprived of Russian gas for close to two weeks.
A spokesman for the SBU security service - the Ukrainian successor to the KGB - confirmed that its agents had moved into the headquarters for the removal of documents as part of a criminal investigation into Naftogaz.
"Today, employees of the SBU went into the headquarters of Naftogaz (in connection with)...a criminal inquiry into the seizure of 6.3 billion cubic meters of gas by Naftogaz personnel," the official said.
An AFP journalist saw four masked men armed with submachine guns, dressed in black, standing in front of the office of one the company's accountants at the Naftogaz building in central Kiev.
The men had arrived at the Naftogaz offices at 1030 GMT.
"They pushed their way through the guards to enter. One has the impression that they are searching and blocking certain offices," said Zemlyansky.
"We suppose they are men from the Alfa special service," which operates under the auspices of the SBU, he said.
The SBU is overseen by Ukrainian President Viktor Yushchenko, who has been engaged in a venomous political feud with Prime Minister Yulia Tymoshenko over the last few months.
"Already yesterday, SBU agents came to Naftogaz and attempted to seize documents including contracts signed with Gazprom," Zemlyansky said.
January's gas crisis was concluded by a contract between Naftogaz and Gazprom after Tymoshenko hammered out a deal with her Russian counterpart, Vladimir Putin, which was sharply criticized by Yushchenko.