Defense Secretary Jim Mattis shakes hands with British Defense Secretary Michael Fallon during a meeting at NATO headquarters in Brussels on Feb. 15, 2017.  (AFP PHOTO / POOL / Virginia Mayo/AFP/Getty Images)

BRUSSELS — Defense Secretary Jim Mattis sought Wednesday to reassure U.S. military allies in the North Atlantic Treaty Organization (NATO), as the Trump administration addressed the fallout of retired Lt. Gen. Michael Flynn’s ouster as President Trump’s national security adviser.

Mattis left Tuesday for a regularly scheduled meeting with NATO defense ministers hours after Trump asked for Flynn’s resignation following revelations that he misled Vice President Pence about secret communications with Sergey Kislyak, the Russian ambassador to the United States. The issue is sensitive for NATO allies, who are concerned about Russian interference in elections and aggression that includes the 2014 annexation Ukraine’s Crimea region.

Mattis, speaking to a throng of reporters alongside NATO Secretary General Jens Stoltenberg, said that the military alliance first established after World War II “remains a fundamental bedrock” of security for both the United States and its allies.

“As President Trump has stated, he has strong support for NATO,” Mattis said. “NATO is in the midst of transformation. It has always been adapting to the security challenges. This is nothing new. Perhaps the pace has changed a bit, but this is something we can deal with.”

Mattis, a retired Marine general who has years of experience working with NATO allies, called Russia the No. 1 threat to U.S. security, but the FBI and the Senate Intelligence Committee are probing ties between other Trump associates and Russia. Trump himself has sent mixed messages about his views on the alliance, saying repeatedly that he views NATO as “obsolete” because it does not do enough to confront terrorism but that Washington will remain committed to it.

Flynn is accused of discussing actions taken by the Obama administration in December in response to intelligence assessments that Russia hacked members of the Democratic Party during the campaign season. He was asked to resign Monday — less than a month into the Trump administration — after it became clear that he misled Pence about doing so.

Trump on Wednesday tweeted that “this Russian connection non-sense” is an attempt to cover up “mistakes made in Hillary Clinton’s losing campaign.” He expressed anger in two other messages that U.S. intelligence is being leaked to the media, and accused then-President Barack Obama of having been “too soft on Russia” in light of Moscow’s annexation of the Crimean Peninsula.

Mattis has been hesitant to discuss Flynn’s ouster, and he took no questions from the media Wednesday. On Tuesday, he took only one question on the record from journalists traveling with him to Brussels. Mattis said the “bottom line” is that he is serving as defense secretary and that Flynn’s departure “has no impact” on his job at the Pentagon.

“It doesn’t change my message at all, and who is on the president’s staff is who I will work with,” Mattis said. “And so it’s full speed ahead and listen, learn, help and lead.”

Stoltenberg, who publicly disagreed in January that NATO is obsolete, appeared to defend the Trump administration on Wednesday when asked about the “chaos” in Washington involving Flynn. The new administration’s message on NATO has been “very consistent,” Stoltenberg said, without drawing a distinction between comments Mattis has made and those from Trump.

“I have spoken with the president twice on the phone. I have spoken with the secretary of defense and the secretary of state, and they have all conveyed the same message to me, as they have conveyed to other leaders in NATO countries,” Stoltenberg said. “And that is that the United States will stay committed to the transatlantic partnership, to the transatlantic bond.”

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