"Right now I would just say there is very little doubt that they have either interfered or they have attempted to interfere in a number of elections in the democracies," Mattis told a press conference.
The US Intelligence Community report claimed that Russia launched an influence campaign in 2016 aimed at undermining public faith in the US democratic process and creating a preference for President Donald Trump.
Russia has repeatedly denied the US allegations calling them absurd and characterizing them as an attempt to divert public opinion from revelations of corruption as well as other pressing domestic issues.
After receiving an intelligence briefing on the US Intelligence Community's report, Trump said the election outcome had not been affected. US House of Representatives Speaker Paul Ryan also suggested that the assessment should not be used by partisans to delegitimize Trump's presidency.