Defense Secretary James Mattis on Thursday said that there was "very little doubt" Russia has attempted to interfere in democratic elections in the past.
"There is very little doubt that they have either interfered or attempted to interfere in number of elections in democracies," Mattis said while answering questions at NATO headquarters in Brussels.
He added that he does not feel compelled to respond to Russian officials who were not pleased with his call to deal with Moscow from a position of strength.
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Mattis maintained that Russia needs to "live by international law" and "prove itself" when it comes to its international commitments.
The defense secretary also reiterated that Washington is fully committed to backing the NATO countries under the Article Five's principle of collective defense.
Mattis on Tuesday threatened to "moderate" U.S. support for NATO if its allies do not increase their contribution.
"The commitment to Article Five remains rock solid. The message that I brought here about everyone carrying their fair share of burden, of sacrifice to maintain the best defense in the world was very well received, it was not contentious," he said.
President Trump's calls for a closer relationship with Russia and criticism of NATO during the campaign and since Election Day have created concerns among NATO nations about the U.S.'s commitment to the treaty.