WASHINGTON — Despite concerns from European partners about America's commitment to NATO, U.S. President Donald Trump has thrown his "full support" behind the alliance, U.S. Secretary of Defense Jim Mattis said Friday.

Mattis, speaking at the Munich Security Conference, also said that Article 5 — the NATO agreement that requires allies to come to the aid of one another — remains a "bedrock commitment" for the United States.

For the last three days, Mattis has been on a goodwill tour of Europe, starting with a NATO ministerial conference, followed by a meeting of the coalition fighting the Islamic State group and finally ending up in Munich on Friday. Across the board, his message has been consistent — that the U.S. remains committed to the alliance, even as the Trump administration is demanding allied countries increase their defense spending.

The relationship between NATO and the new U.S. president has been tense ever since then-candidate Trump hinted in a July interview that his support for NATO nations would be conditional based on whether those countries had "fulfilled their obligations to us."

While Mattis has downplayed such comments, he did worry some in Europe with his Wednesday comments that allies must start to increase defense spending by year's end or the Trump administration will "moderate its commitment" to them.

Asked to define what "moderate" meant, Mattis told reporters after his speech that the message was not a new one, and that European leaders understand where the administration is coming from.

"The message that I brought here about everyone carrying their fair share of the burden, the sacrifice to maintain the best defense in the world, was very well received," Mattis said, according to a transcript posted by the Pentagon. "It was not contentious. There was no argument; there was simple discussion about how best and how fast can each nation with its own particular circumstance reach it. I leave here very optimistic."