New Pentagon chief James Mattis said Wednesday that NATO was a "fundamental bedrock" for the United States as he  tried to reassure allies concerned about President Donald Trump's commitment.

"The alliance remains a fundamental bedrock for the United States and for all the transatlantic community, bonded as we are together," Mattis said as he met his counterparts in Brussels for the first time.

"And as president Trump has stated, he has strong support for NATO," added  the former marine general, who has himself previously served with the alliance.

"I am here to listen to my fellow ministers, to have an open conversation among friends and allies, about where we're going and our shared level of commitment."

NATO allies have expressed worries about Trump's comments as president-elect that the alliance was "obsolete," as well as his apparently softer stance on Russia.

Mattis however stressed that the Trump administration, like the Obama administration before it, expected NATO allies to boost their defence spending.

"It's absolutely appropriate, as a European minister of defence said last week -- it's a fair demand that all who benefit from the best defence in the world carry their proportionate share of the necessary cost to defend freedom," he added. (AFP)