"Based on the fact that the ships and sailors have not been returned to Ukraine from Russian Federation, I have decided it would be best for all parties concerned to cancel my previously scheduled meeting in Argentina with President Vladimir Putin", Trump said.
Last week, US Ambassador Nikki Haley to the United Nations accused Russian Federation of "outlaw actions" and an "outrageous violation of sovereign Ukrainian territory" while addressing an emergency meeting of the Security Council.
"This is a very complex situation because Mr. Putin is clearly a slow learner", Mattis said.
Asked if U.S. -Russian ties had become more strained since he took over the Pentagon past year after Trump became president, Mattis said, "There's no doubt the relationship has worsened".
"What we witnessed this weekend is yet another reckless Russian escalation" of what began with Russia's seizure of Crimea from Ukraine in 2014 and its ongoing support for separatists in eastern Ukraine, said outgoing U.S. Ambassador Nikki Haley in response to the actions.
But independent researchers and social media firms countered this conclusion, saying that Russian Federation has indeed continued its online influence campaigns to sow distrust in American democratic institutions.
While Trump spoke briefly with Putin on the sidelines of the summit, he chose to cancel the two leaders' formal meeting. "He is not acting in the best interest of the Russian people".
"But we do not accept predatory economic practices or coercion of smaller states", Mattis said. He was referring to the 1987 Intermediate-range Nuclear Forces accord between Washington and Moscow that the White House has indicated it intends to withdraw from soon.
Mattis, in a speech at the Reagan Presidential Library and during a question-and-answer session afterward, was unyielding in his criticism of Putin.