Defense Secretary Jim Mattis said Thursday at NATO headquarters he has “little doubt” Moscow has tried interfering in democratic elections, while also rebuffing calls for renewed military cooperation with Russia.

"Russia is going to have to prove itself first," Mattis told reporters, adding it is premature to talk about cooperating militarily with Moscow.

Amid rising evidence that Russia has begun interfering in several upcoming European elections using a strategy of leaked emails and ‘fake’ news, Mattis gave a blunt response when asked if he thinks Russia did the same thing in the U.S. election.

"There is very little doubt they have either interfered or attempted to interfere in elections of democracies," Mattis said in Brussels.

The tough comments came as Chairman of the Joint Chiefs Gen. Joe Dunford flew Thursday to Baku, Azerbaijan, to meet with Valery Gerasimov, his Russian counterpart -- the first meeting of top Russian and American military commanders since Russia invaded Ukraine three years ago.

Russia's defense minister separately chastised Mattis for rebuffing the proposal for renewed cooperation, saying if the U.S. thinks it can negotiate from a position of strength, then talks are hopeless.

"I have no need to respond to the Russian statement at all. NATO has always stood for military strength," Mattis said, when asked about the Russian defense minister's remarks.

The comments reflect lingering reservations from some in the Trump administration about interactions with Russia, even as President Trump’s team faces accusations of being too cozy with Moscow.

"Russia sees the world as a very zero-sum game. The only way that Russia can ascend is if the U.S. declines," NATO's former Supreme Allied Commander Retired General Phillip Breedlove warned.

Breedlove, who first raised alarm bells about Russian actions in Europe and ordered U.S. tanks and troops to preposition in Europe for the first time since the Cold War, also described in a sit-down interview with Fox News how his emails were hacked and released by Russia in an effort to discredit him.

"My email was hacked and placed out on DC Leaks," Breedlove said. "Those emails were contrived by trolls -- the many trolls in Russia who work on me."

The emails portrayed him and former Gen. Colin Powell colluding to overthrow the Obama government. "I mean it's fantastical some of the things that were created from that episode," he said.

Breedlove warned that Russian efforts to undermine elections in the West would not stop, adding there is evidence the Russians are still operating in the U.S.

"They are just trying to discredit our form of democracy. To convince the people in the United States that what they just went through was not legitimate and cause them to lose faith in the democratic system," he said.

The former NATO commander was referring to Russian hacking and information operations undermining Western democracy by targeting their elections.

"I believe they are doing the same thing in many European countries,” he said.

Gerasimov, whom Dunford met with in Baku Thursday, has even written about these kinds of operations and describes them as "indirect war," writings Breedlove cited as he discussed Russian efforts to “discredit” American democracy.

“This is written about in Russian lexicon now. They call this war by indirect means,” he said. “I call it war below the lines -- doing things in a nation that would remain below that threshold of more strenuous response. I think they are doing it all around.”