U.S. defence secretaries routinely visit North Atlantic Treaty Organisation, but Mattis's trip is significant as he seeks to reassure allies - rattled by Trump's past rhetoric on the alliance - that America is not abandoning long-standing security doctrine to forge closer ties with Moscow.
As he entered the meeting, NATO Secretary General Jens Stoltenberg avoided a question about Flynn, saying it's a crucial time in the trans-Atlantic alliance and neither Europe nor the US can tackle their challenges alone.
NATO Secretary General Jens Stoltenberg addresses a press conference during a NATO Defense Ministers Meeting at its headquarters in Brussels, Belgium, on February 15, 2017.
His remarks caused consternation among the allies who, stung by Russia's intervention in Ukraine, had agreed in 2014 to increase defense spending to two percent of national economic output by 2024, reversing years of cuts. "I believe that it provides useful principles for a dialogue with Russia", Stoltenberg said.
Britain's Defence Secretary Michael Fallon asked members that are now spending less than two percent of GDP on defense to "at least demonstrate good faith" by committing to an annual increase.
Amid these tensions, Stoltenberg seemed please that the new USA defense secretary did not repeat President Trump's remarks that the alliance had become obsolete.
The British Government has rejected claims a think tank's calculation that the UK's spending on defence has dipped below Nato's two per cent target.
NATO's European allies increased military spending by almost four percent previous year, bucking a recent trend of cuts.
The Giants and the London Trip: Great Opportunity to Build Bonds
Of course, the 2007 trip wasn’t very conducive; the team went to the practice field the moment they landed in London. The NFL is a little more complicated; there are always new faces coming into the fold, new rookies and free agents.
Trump "has strong support for Nato", he said, against a backdrop of fresh scrutiny over the new USA administration's alleged links to Russian Federation.
From August 2010 to March 2013, Mattis, then a Marine general, led an investigation into unauthorized disclosures of classified information allegedly made by Flynn, who was then a lieutenant general in the US Army.
But the resignation of Michael Flynn as USA national security adviser - and the rumbling row over the Trump team's contacts with Russian Federation - is likely to cause unease among the 28-nation group, our correspondent adds. "I'm confident that defense ministers at our meeting tomorrow and the day after tomorrow once again are going to reconfirm the enduring importance of the transatlantic bond". "It amounts to roughly 10 billion dollars more for our defense", he said.
"Neither Europe nor America can tackle them alone".
The NATO Secretary General Jens Stoltenberg has said the alliance is firmly committed to its Resolute Support mission in Afghanistan.
He said that the defence spending will be one of the main issues to be discussed at the meeting.
"He (General Mattis) said the patience of the United States taxpayer is beginning to run out - why should they take a greater share of the burden when all of us have committed to 2%".