Ukraine reported a targeted hack of government websites Friday amid a deepening crisis with Russia and left experts puzzling over the ominous message left by the hackers: “Be afraid and expect the worst.”

Ukraine officials said it was too early to say who was behind the hack — which Ukraine described as “massive” — but noted Russia had been behind similar attacks before. Analysts said the defacements may be the work of non-state agents, or hacktivists, noting that vandalizing government websites was not a sweeping or sophisticated hacking operation.

But at least one Ukrainian agency, the Center for Strategic Communications and Information Security, openly blamed Russia, linking it to Russia’s efforts to block Ukraine’s aspirations to join NATO.

In a statement, Ukraine’s cyber police said that 10 of the roughly 70 sites that were attacked experienced an “intrusion” but that the sites’ contents were “not altered and no personal data was leaked.”

Officials also said that there was a “high probability” it was a “supply-chain attack” in which hackers first entered through the system of a commercial company.

The timing of the attack also elevated worries in Ukraine. It came a day after the latest round of diplomatic efforts in Europe failed to deter Russia’s military buildup near Ukraine. Russia stood firm on its demands, including that NATO block Ukraine from possibly joining the military alliance and end military aid to Ukraine.

The Ukraine hack also triggered concern in Washington and Europe with officials watchful of the role of cyber and information attacks in modern warfare. Any major cyberattack on Ukraine by the Russian state could also trigger tough new sanctions.

Ukraine’s cyber police said that “more than five” government sites were attacked and that authorities had launched an investigation to identify the perpetrators. Officials said it was too early to say who was behind the attacks.