US commitment to NATO 'iron-clad': Jim Mattis

US Defense Secretary Jim Mattis on Tuesday sought to reassure European allies rattled by President Donald Trump's rhetoric, insisting that America's commitment to NATO is "iron-clad".

At a press conference in Paris, Mattis said "actions speak loudest", adding: "The US has acted, showing that our commitment to the transatlantic alliance remains iron-clad."

He stressed that US lawmakers have not cut military spending in Europe "by a single cent, instead maintaining once again the highest levels of commitment since the 1989 fall of the Berlin Wall".

Some NATO members are concerned about Trump's commitment to the alliance and have bristled at his repeated criticism of European allies for not spending enough on their militaries, particularly Germany.

Parly, speaking alongside Mattis at the French defence ministry, told reporters that "Europe is not part of the problem, Europe is part of the solution".

"The United States is making sharing the burden a priority. It is also a French priority," she said. 

"It's a question of making the whole Atlantic alliance function better."  

Mattis' words of reassurance come a day ahead of a gathering of NATO defence ministers in Brussels.

US officials have stressed that Washington is fully committed to NATO and mutual defence, despite comments from Trump that have worried some of its members.

Before taking office he called NATO "obsolete" and soon after a tumultuous summit last July he questioned whether the US would honour the alliance's founding principle of mutual defence for newest member Montenegro.