Kyiv has appeared to point the finger of blame at Moscow.
ADVERTISEMENTUkraine says that its defence ministry and two state-owned banks have been hit by a cyberattack.
The ministry confirmed on social media that it had "probably" been targeted and was working to restore its website.
Meanwhile, Ukrainian authorities said that the Armed Forces website, Privatbank, and Oschadbank had also been disrupted on Tuesday evening.
"Privat assures that there is no threat to depositors' funds," according to a statement by Ukraine's Centre for Strategic Communications and Information Security.
Netblocks -- an internet monitoring observatory -- confirmed that the websites had been disrupted "in a manner consistent with a denial of service (DDOS) attack".
It added that the initial attack began on Tuesday afternoon before intensifying over the course of the day.
"Some service was restored by the evening," Netblocks said in a statement.
Ukraine did not name any suspects behind the attack but has appeared to point the finger at Moscow, amid heightened tensions of troop buildups.
"It is possible that the aggressor used little dirty tactics because its aggressive plans are not working out on a large scale," the Ukrainian authority said.
On Wednesday, Kremlin spokesperson Dmitry Peskov denied any Russian involvement in the cyberattacks and said it "predictable that Ukraine would continue to accuse Russia of everything".
It is not the first time that Kyiv has blamed Moscow for similar cyberattacks in recent weeks.
Last month, dozens of Ukrainian government websites were targeted in a website-defacing attack that warned Ukrainian citizens to “be afraid and expect the worst”
Ukraine blamed that attack on intelligence officials in Russia and its ally Belarus -- allegations that have been dismissed.
NATO had said that it would work with Ukraine to improve the countries' cyber defences. The European Union also said last month that it was mobilising "all its resources" to help Kyiv.