The North Atlantic Treaty Organization (NATO) confirmed a "non-military" attack against it on Sunday evening, and while fingers are pointing at Russia, there was no comment from Moscow on the matter.
"NATO cyber experts are actively responding to a cyber attack that affected some of the alliance's websites," a NATO official told dpa.
"NATO deals with cyber incidents on a regular basis, and takes cyber security very seriously," he added.
The comment from the official came after reports on social media indicated that pro-Russian hackers had targeted the website of NATO's operational headquarters and others, making it temporarily inaccessible.
The Russian hacking group Kelent has been accused of being involved in the attack, among other groups.
The group has been linked to other recent attacks, including in Germany, targeting the German Parliament (Bundestag), police and critical infrastructure facilities.
Moscow constantly says that it is in "direct war" with NATO, which it accuses of waging a proxy war against it in Ukraine by supporting and arming Kiev, according to it, since the outbreak of the conflict last February.