US commitment to NATO 'unwavering' despite Trump: Kerry

US Secretary of State John Kerry (R) listens to US ambassador to the NATO Douglas Lute during a meeting in Brussels. (AFP/JOHN THYS)

"The change of the administration will not change the unwavering commitment of the US to ... our NATO obligations," Kerry said after talks with his NATO counterparts in Brussels.

"The US commitment to NATO and Article Five transcends politics," added Kerry, referring to the military alliance's collective defence policy in which an attack on one member is an attack on all.

Trump stoked concerns that Washington's near 70-year European defence guarantee might no longer hold when he said on the campaign trail he would think twice about helping NATO allies who did not pay their defence dues.

His commitments particularly alarmed eastern European NATO countries that are on edge about a more assertive Moscow following Russia's annexation of Crimea and involvement in the conflict in Ukraine.

But Kerry insisted that both the Trump administration and the US Congress would continue to back NATO.

"I'm absolutely confident that the next administration and Congress will stand together," Kerry said in his swansong to Europe.

"I am confident Article Five is not going to be held hostage to the lack of one country or other (to meet its defence spending commitments) ... Article Five is much larger, it is about principle."

The top US diplomat also gave his backing to Trump's pick for defence secretary, retired general James "Mad Dog" Mattis.

"I am confident in some of the people I have seen so far, General Mattis ... I am certain they will remain committed to the core components of the transatlantic alliance," Kerry said.

But he insisted he had no idea of his likely successor as secretary of state.

"The list of people replacing me is growing," Kerry said, with names bandied about for weeks including former Trump critic Mitt Romney, one-time CIA director David Petraeus, former New York mayor Rudy Giuliani, Senate Foreign Relations Chairman Bob Corker or even former UN ambassador John Bolton.

Kerry meanwhile insisted that the historic nuclear deal with Iran, which Trump promised during his campaign to tear up, was worth keeping. "The Iran nuclear agreement has made the world safer," he said.

Overall Kerry said that the world should wait to see what Trump actually does and "must not get all churned up over things that have not happened or appointments that have not been named." "Common sense will prevail," he added.