The US Secretary of Defense also supported London and Amsterdam in their claims against Moscow.
"I’ve seen enough of the evidence to say the Dutch and British are 100 percent accurate in who they have attributed this to," Mattis said.
Earlier on Thursday, the UK Foreign Office said in a statement that London and its allies could expose a GRU campaign involving erratic and reckless cyberattacks on political institutions, enterprises, media and sports. UK National Cyber Security Centre (NCSC) cited technical details, according to which the GRU was "almost certainly" involved in the theft of confidential medical documents of several athletes published in August 2017 by WADA.
NATO Secretary General Jens Stoltenberg said the organization stands in solidarity with London and Amsterdam in their accusations and urges Moscow to change its behavior. NATO will respond by strengthening its defense against hybrid threats and cyber attacks, Stoltenberg added.
Russian Foreign Ministry Spokesperson Maria Zakharova said all the allegations are simply the start of yet another international anti-Russia campaign.
Western officials put forward accusations against "Russian hackers" on a regular basis. Russia has repeatedly refuted allegations of interfering in other countries' internal affairs and elections.
INF Treaty Issues
Mattis also stated that the United States was reviewing all diplomatic and defense options should Russia not return to compliance with the Intermediate-Range Nuclear Forces (INF) Treaty.
"As [US] Ambassador [to NATO Kay Bailey] Hutchison has made clear, Russia must return to compliance with INF treaty or the US will need to respond to its cavalier disregard of the treaty’s specific limits. The United States is reviewing options in our diplomacy and defense posture to do just that," he told reporters.
The 1987 INF Treaty prohibits the development, deployment, and testing of ground-launched ballistic or cruise missiles with ranges between 300 and 3,400 miles. Moscow and Washington have repeatedly accused each other of violating the treaty.
According to the US, the new Russian 9M729 missile systems violate the conditions of the pact, giving Russia the possibility to launch a nuclear strike in Europe with little or no notice, while Moscow stated that the United States is violating the treaty by deploying in Europe missile defense systems with launchers, which might be used for firing Tomahawk cruise missiles.