Defense Secretary James Mattis James Norman MattisBiden courts veterans amid fallout from Trump military controversies Trump says he wanted to take out Syria's Assad but Mattis opposed it Gary Cohn: 'I haven't made up my mind' on vote for president in November MORE said Saturday that Russian operatives attempted to interfere in the 2018 midterm elections, apparently confirming for the first time that Moscow attempted to meddle in last month's elections.

Mattis spoke of the relationship between the Trump administration and Russian President Vladimir Putin during an interview Saturday at the Ronald Reagan Presidential Library in California.

"There is no doubt the relationship has worsened. He tried again to muck around in our elections this last month," Mattis said. "We are seeing a continued effort around those lines."

Mattis said the Trump administration and other NATO allies have pressed Russia repeatedly on un-democratic actions, but to no avail.

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"This is a very complex situation because clearly Mr. Putin is a slow learner," Mattis said. "He is not recognizing that what he is doing is actually creating the animosity against his people."

"We're dealing with someone we simply cannot trust," he added.

Mattis's comments come after President Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE canceled a meeting with Putin at the Group of 20 (G-20) summit in Argentina this week, citing a naval dispute between Ukrainian and Russian forces on the border of Crimea, which raised fears of new hostilities in the region.

Russian officials fired back on Friday, accusing Trump of canceling the meeting due to domestic political troubles for the Trump administration, and not on any actions taken by Russia toward Ukraine.

The cancellation of the meeting came the same day that the president's former lawyer, Michael Cohen, pleaded guilty to making misstatements to Congress about Trump's business dealings in Russia, while agreeing to cooperate further with Robert Mueller Robert (Bob) MuellerCNN's Toobin warns McCabe is in 'perilous condition' with emboldened Trump CNN anchor rips Trump over Stone while evoking Clinton-Lynch tarmac meeting The Hill's 12:30 Report: New Hampshire fallout MORE's special counsel investigation.

Trump maintained on Friday that the incident in Ukraine was the sole reason for the cancellation of his planned meeting with Putin.

“We don't like what happened, we're not happy about what happened,” Trump told reporters while meeting with Japanese Prime Minister Shinzo Abe. “On the basis of what took place with the ships and sailors, that was the sole reason."

The White House said Saturday that Trump and Putin had an "informal" conversation during a dinner the previous night for global leaders at the G-20 summit. The White House did not say what the two leaders discussed.