A cyberattack aimed at paralyzing Ukraine’s top banks and government websites was the worst in the country’s history as its standoff with Russia continues to draw warnings of a potential invasion.

The DDoS, or distributed denial-of-service, attack began Tuesday and continued into Wednesday with the goal of causing mass confusion, Mykhailo Fedorov, Ukraine’s minister of digital transformation, said during an online briefing.

“This attack was unprecedented, it was prepared well in advance, and its key goal was destabilization, sowing panic, and creating chaos in our country,” he said.

The Defense Ministry’s website and Dia, a government service that lets people access digital versions of their passports and COVID vaccination certificates, were among the targets. Cash machine networks and mobile banking services of top lenders Oshchadbank and Privatbank also came under pressure.

The size and resources involved suggest “a country” was behind the attack, according to Viktor Zhora, the deputy head of the State Service of Special Communication and Information Protection.

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He didn’t name the country, citing an ongoing investigation. In the past, Ukraine’s Security Service has said hacking groups linked to Russian intelligence services may have been responsible for similar attacks, allegations Moscow has denied.

Kyiv’s western allies say Russia has massed as many as 150,000 troops on its neighbor’s borders in a possible precursor to an attack and are voicing reservations about announcements from the Kremlin that it is withdrawing some forces. Russia, which annexed Crimea from Ukraine in 2014 and supports separatists in Ukraine’s eastern Donbas region, says it has no plans to invade.

The attack was “purely psychological,” as it sought to prevent people from accessing services and didn’t appear to steal any money or information, Deputy Secretary of the National Security Council Serhii Demedyuk said.

While Ukrainians are increasingly reliant on using mobile banking and digitalized government services, they are also accustomed to interruptions from hackers.

In 2015, a cyberattack cut electricity to 200,000 customers, while another a year later temporarily knocked out a power station in northern Kyiv. Those attacks were carried out with involvement from Russia’s GRU intelligence agency, according to charges the U.S. Department of Justice unsealed in 2020.

This year, hackers defaced Ukrainian government websites in January, broadcasting a message that falsely claimed to have stolen private data from citizens.

Russia “had nothing to do” with this week’s cyberattack in Ukraine, Kremlin spokesman Dmitry Peskov said. He described as “inaccurate” assertions from NATO that a pullback of Russian troops from near Ukraine’s border is not taking place.