The Netherlands is accusing Russia of carrying out multiple cyberattacks, including one against the Organization for the Prohibition of Chemical Weapons, which is based in The Hague.
Dutch officials said Thursday they caught four members of Russia's military intelligence service with spy equipment next to the OPCW headquarters in April and expelled them.
The OPCW was investigating the poisoning of a former Russian spy in Britain. Russia has denied any links to the poisoning of Sergei Skripal and Dutch officials said it was not clear if the cyberattack was linked to the investigation.
Britain and Australia are also accusing Russia of malicious cyber activity, saying its agents targeted the World Anti-Doping Agency and the 2016 U.S. presidential election.
Russia's Foreign Ministry denied the accusations Thursday, dismissing them as "fantasies."
NATO Secretary General Jens Stoltenberg warned Russia to stop its "reckless" behavior and said NATO allies will support the British and Dutch governments.
"NATO allies expressed their solidarity with the decision by the Dutch and the British governments to call out Russia on its blatant attempts to undermine international law and international institutions," Stoltenberg said at a news conference Thursday in Brussels, where he is meeting with counterparts from the alliance.
"This aggressive act demonstrated contempt for the solemn purpose of the Organization for the Prohibition of Chemical Weapons, which works to eradicate weapons worldwide under a United Nations mandate," said a statement from European Union Commission President Jean-Claude Juncker, EU Council President Donald Tusk and EU foreign policy chief Federica Mogherini.
British Foreign Secretary Jeremy Hunt also put Moscow on alert, saying, "The Russian government needs to know that if they flout international law in this way, there will be consequences, they will be exposed."
U.S. Defense Secretary Jim Mattis told reporters in Brussels that Russia must be held accountable for its actions. Mattis said a number of retaliatory actions against Russia were could be considered, including the provision of U.S. cyber warfare capabilities to NATO.