Russian forces opened fire on Ukrainian naval vessels last weekend and seized them and their crew near Crimea, which Russia annexed from Ukraine in 2014. Mattis said those actions underscored why the international community was increasingly distrustful of Moscow.
"Mr. Putin is clearly a slow learner. He is not recognizing that what he is doing is actually creating an animosity against his people," Mattis said.
"What we are seeing Putin do with his ripping up of international agreements ... we're dealing with someone that we simply cannot trust," the Pentagon chief added.
White House spokeswoman Sarah Sanders on Saturday confirmed Trump's encounter with Putin in Argentina in a statement, saying, "As is typical at multilateral events, President Trump and the First Lady had a number of informal conversations with world leaders at the dinner last night, including President Putin."
Speaking in Buenos Aires, Putin told reporters on Saturday there were no pre-conditions for future bilateral talks with Trump.
"It is regrettable that we can't succeed in holding a full-scale meeting, which is long due," Putin said, adding that issues of strategic stability would be of paramount importance.
U.S. Secretary of State Mike Pompeo on Saturday said the Ukraine events were the only reason Trump canceled his planned formal meeting with Putin in Argentina, not developments in Special Counsel Robert Mueller's Russia probe.
The decision to scrap the meeting with Putin came shortly after Trump's former personal lawyer Michael Cohen pleaded guilty on Thursday to a charge of lying to Congress about a skyscraper project Trump was pursuing in Moscow during the 2016 U.S. presidential race.
"Ludicrous; Washington parlor game," Pompeo said in a CNN interview at the G20 meeting when asked whether Trump was motivated to cancel the meeting by Cohen's guilty plea.
