"As a result of the cyberattack, the exchange of information between the units of the Ministry of Defence of the Russian Federation, which used the indicated server located in Moscow, was stopped," Ukraine's GUR military intelligence unit said in a statement.

It added that the cyberattack was "ongoing".

AFP was unable to verify the claim, and Russian officials had not immediately responded to the alleged hacking.

But Russian authorities did announce Tuesday that a "technical problem" had shut down several internet sites in the country, though the defence ministry was not mentioned.

Russia's digital development ministry said the problem was linked to "Domain Name System Security Extensions" and was resolved by Tuesday evening, though "disruptions" could persist "for a certain time".