The U.S. defense secretary strongly criticized Russian President Vladimir Putin, accusing him of deceit involving a key Cold War-era arms control treaty, and acting recklessly in last week’s naval confrontation with Ukraine.

In his comments December 1, Jim Mattis also asserted that Moscow had tried to interfere in last month’s midterm congressional elections.

"We are dealing with someone that we simply cannot trust," Mattis said during a speech at the Reagan National Defense Forum in California. "There is no doubt the relationship has worsened."

Mattis was also asked how Washington could deter further Russian confrontation.

"This is a very complex situation because Mr. Putin is clearly a slow learner," Mattis said. "He is not recognizing that what he is doing is actually creating the animosity against his people. He's not acting in the best interests of the Russian people, and he is actually causing NATO to rearm.”

The U.S. defense chief also made passing reference to the Intermediate-Range Nuclear Forces treaty, a 1987 agreement known as the INF, that Washington has accused Moscow of violating.

Moscow has denied the accusations, though last year, it did acknowledge the existence of a missile that Washington had identified, but it insisted the missile did not violate the treaty.

"We are dealing with Putin's duplicitous violation of the INF Treaty," Mattis said.

Mattis did not make reference to President Donald Trump’s recent announcement that the United States would be withdrawing from the treaty.

Referring to last week’s naval confrontation near the Crimean peninsula, in which Russian ships seized three Ukrainian ships and two dozen Ukrainian sailors, Mattis criticized what he called Russia’s "brazen" violation of a treaty with Ukraine.​

Mattis also asserted that Russia had interfered in the November midterm elections for Congress, though he gave no details nor provided any further evidence.