Ukraine's State Security Service (SBU) raided the local offices of Russia's top search site Yandex on Monday in an operation that SBU spokesman Olena Gitlyanska said was part of a treason investigation.
Earlier in May, Ukraine imposed sanctions on Yandex and several other Russian internet firms and blocked access to their websites, saying it wanted to guard against cyber threats.
"There are representatives of Ukraine's Security Service in our offices in Kiev and Odessa," Yandex said in a statement, saying its lawyers were willing to cooperate with any investigation.
In a Facebook post, the SBU's Gitlyanska said "investigative actions" were being carried out in connection with a criminal case into suspected treason, but did not give further details.
A Reuters witness at Yandex's Kiev office saw plain-clothes investigators sticking yellow labels onto a number of computers that appeared to have been seized.
More than 450 Russian firms have been blacklisted by Kiev since Moscow's annexation of Crimea in 2014 and the ensuing pro-Russian separatist uprising in eastern Ukraine.
Yandex has previously said it operates fully in accordance with Ukrainian law. It does not expect sanctions to have a material negative impact on its business.
Before sanctions were imposed, Yandex employed 320 people in Ukraine and had around 11 million Ukrainian users.
(Reporting by Natalia Zinets and Pavel Polityuk in Kiev; Anastasia Teterevleva in Moscow; Writing by Alessandra Prentice; Editing by Richard Lough)