Last year, Trump repeatedly told senior officials that he did not see the point of NATO -- the historic alliance that forms the backbone of the West's post-World War II security order -- and that he wanted to withdraw, The New York Times reported.

He has often blasted members of the 29-nation partnership for not paying more into their national defense budgets.

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

Eric Pahon, a Pentagon spokesman, said the US remains "100 percent" committed to NATO.

At the summit the president said the US "commitment to NATO is very strong" and "tremendous progress has been made" by allies and partners.

"That has not changed," Pahon said in a statement.

"NATO remains the cornerstone of transatlantic security."

In Brussels, a NATO official also highlighted Trump's comments from the July summit.

"The United States is strongly committed to NATO and to transatlantic security," the official told AFP.

"The US has significantly boosted its commitment to the defense of Europe, including with increased troop commitments."

Turning 70 this year, NATO has underpinned Western security in Europe for decades, first countering the Soviet Union and then Russian expansionism.

A US withdrawal from NATO would be a strategic gift of epic proportions to Russia, which is accused of meddling in the 2016 presidential elections to help Trump win.

Former defense secretary Jim Mattis was a staunch proponent of NATO and repeatedly visited its Brussels headquarters, where he sought to reassure allies about America's commitment to the alliance.

But Mattis quit last month, and observers see a shrinking coterie of advisors around Trump willing to push back against him.

The US Congress, including Trump's own Republican Party, would likely push back against any effort to withdraw from NATO.

The only country to have ever invoke Article 5, NATO's collective defense principle, was America following the September 11, 2001 attacks.