Originally Published: 27 JUN 22 12:10 ET

By Sean Lyngaas, CNN

(CNN) -- An "intense, ongoing" cyberattack has hit the websites of government agencies and private firms in Lithuania, the Baltic country's defense ministry said Monday.

A Russian-speaking hacking group, known as Killnet, claimed responsibility for at least some of the hacks, saying they were in retaliation for Lithuania blocking the shipment of some goods to the Russian enclave of Kaliningrad, which is wedged between Lithuania and Poland.

Monday's cyberattacks were aimed in part at Lithuania's Secure Data Transfer Network, a communications network for government officials that is built to withstand war and other crises, according to the defense ministry.

"Part of the Secure National Data Transfer Network users have been unable to access services, work is in progress to restore it to normal," Lithuania's National Cyber Security Centre (NKSC) said in a statement issued by the defense ministry.

"It is highly probable that such, or even more, intense attacks will continue into the coming days, especially against the communications, energy and financial sectors," acting NKSC Director Jonas Skardinskas said in a statement.

The type of hack in question is known as a distributed denial of service attack, which floods websites with phony traffic to knock them offline. It's a common tool favored by "hacktivists" like Killnet -- suspected non-state actors who conduct cyberattacks for political causes.

Since Russia's full-fledged invasion of Ukraine in February, a bevy of pro-Ukraine and pro-Russian hacking groups have caused disruptions to an array of organizations in Ukraine and Russia. A group known as the Belarusian Cyber-Partisans, for example, has claimed hacks on the IT systems that support trains moving Russian soldiers near the front lines in Ukraine.

Lithuania is a member of NATO, the 30-country alliance that includes the US, which has been on guard for pro-Russian hacks for months. CNN has requested comment from NATO on Monday's hacking incidents.