Ukrainian national security service agents have raided the head office of the state energy firm Naftogaz in Kiev.
The raid by armed agents was linked to an investigation into the alleged diversion of gas worth $880m (£620m), a spokeswoman told the Associated Press.
Earlier, a Naftogaz spokesman said armed men were on the building's third floor, near a deputy director's office.
Naftogaz was at the heart of a dispute with Russia in January that led to gas shortages for millions across Europe.
Russia suspended gas supplies to Ukraine for two weeks until Ukraine agreed prices at which they would buy Russian gas, and ship it to Europe. Previously, Ukraine had paid a heavily discounted rate.