Donald Trump's foreign policy proposals would make the world a less stable place, former Secretary of State James Baker told a U.S. Senate hearing on Thursday as the Republican presidential candidate met elsewhere with party congressional leaders.

Under questioning from Republican Senator Marco Rubio, a former Trump rival in the presidential race, Baker said the world "would be far less stable" with a weaker NATO or if more countries had nuclear weapons as Trump has proposed.

"We've a got a lot of problems today, but we'd have a hell of a lot more if that were the case," Baker told a Senate Foreign Relations Committee hearing, adding that U.S. commitments around the world "promote U.S. security."

Trump met with Baker on Thursday at Trump's request, said a Baker spokesman, who declined further comment.

The hearing, on "America's Role in the World," was called by the committee's Republican chairman, Senator Bob Corker. Corker praised a foreign policy speech Trump gave in Washington last month. Some U.S. allies worried after Trump's remarks that his invocation of an "America first" agenda is a threat to retreat from the world.

Without naming Trump, Rubio referred to the businessman-turned-candidate's suggestions that the United States should rethink the North Atlantic Treaty Organisation (NATO) and that Japan and South Korea should consider getting nuclear weapons to defend themselves.

"Some have suggested 'why don't you just let Japan and South Korea get their own nuclear weapons and let them defend themselves?'" Rubio asked.

"The more countries that acquire nuclear weapons, the more instability there is going to be in the world, in my opinion," Baker said.

Tom Donilon, Democratic President Barack Obama's former national security adviser, called Rubio's question an "important thought experiment," as he backed Baker's comments about the importance of NATO.

"It's not just a thought experiment, it's actually been proposed," Rubio said.

As the hearing took place, Trump was on Capitol Hill meeting with Republican congressional leaders on how to heal divisions within the party, including those between establishment figures like Baker and the insurgent candidate.

Baker, a Republican who was secretary of state under President George Bush and Treasury secretary under President Ronald Reagan, testified alongside Donilon.

Former Presidents Bush and George W. Bush do not plan to endorse Trump, or any candidate, in this year's White House race.