"We will be with NATO 100 percent, but as I told the countries, you have to step up and you have to pay," Trump said in a speech at the Pentagon.

His comment follows a report in The New York Times that Trump last year spoke to senior officials about pulling the United States from the key western security alliance.

Trump has often publicly 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 last year, he questioned whether the US would honor the alliance's founding principle of mutual defense for newest member Montenegro.

On Thursday, Trump repeated his view that close allies had been taking advantage of the US security umbrella for decades and that it was his mission to stop that.

"We cannot be the fools for others. We cannot be. We don't want to be called that. And I will tell you for many years behind your backs, that is what they were saying," Trump said.

Trump was speaking during an address to the Pentagon focused mostly on the planned expansion of US missile defense capabilities that the president said would counter emerging threats.

These include Russia's recently unveiled hypersonic missiles, which Moscow says cannot be stopped by any kind of interceptor.

Trump said expanded US missile defenses will be "unrivaled and unmatched."