The resignation of President Donald Trump's national security adviser, Michael Flynn, will have "no impact" on America's message to worried NATO partners, Pentagon chief Jim Mattis said on Tuesday ahead of a key alliance meeting.

Flynn was forced from his key post late Monday amid allegations he had discussed US sanctions strategy with Russia's ambassador Sergey Kislyak before taking office.

US defence secretaries routinely visit NATO, 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.

"Frankly, this has no impact," Mattis told reporters as he flew to Brussels for a meeting of NATO defence ministers on Wednesday and Thursday.

"Obviously I haven't changed what I am heading there for. It doesn't change my message at all, and who is on the president's staff is who I will work with," Mattis said.

Flynn's departure marks a dramatic development in Trump's presidency and comes amid broad international concerns over Moscow's alleged meddling in foreign elections and Trump's friendliness towards Russian President Vladimir Putin.

And as a candidate, Trump made it clear that he has no sentimental attachment to NATO, arguing that European members don't pay their fair share and calling the alliance "obsolete."

Since his election, Trump has moderated his criticism and appointed in Mattis a strong supporter of allied cooperation.

Mattis, a retired four-star Marine Corps general, has worked extensively with international partners in Iraq and Afghanistan.

Unlike his boss, he is a fierce NATO advocate, and has said that if the alliance did not already exist, it would need to be created.

"This has been the most successful alliance in military history," he told reporters, praising NATO for its enduring role in helping the United States in Afghanistan.