US president Donald Trump's defence secretary plans to visit Japan and South Korea next week, choosing the two close US allies for his debut trip abroad as Pentagon chief, a US official said on Tuesday, speaking on condition of anonymity.

James Mattis, a retired Marine general, was sworn into the Pentagon's top job shortly after Trump's inauguration on Friday and a quick departure to Asia could be seen as a nod to the importance the new US administration places on security ties.

The trip would closely follow Mr Trump's withdrawal from the 12-nation Trans-Pacific Partnership (TPP) trade pact, fulfilling a campaign pledge but disappointing many key US allies in Asia.