WASHINGTON (Reuters) - U.S. President Donald Trump’s defense secretary plans to visit Japan and South Korea next week, choosing the two close U.S. allies for his debut trip abroad as Pentagon chief, a U.S. 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 U.S. administration places on security ties. The trip would closely follow Trump’s withdrawal from the 12-nation Trans-Pacific Partnership (TPP) trade pact, fulfilling a campaign pledge but disappointing many key U.S. allies in Asia. Japanese Prime Minister Shinzo Abe, for example, had touted TPP in part as a counter-weight to a rising China. No further details were immediately available about Mattis’ proposed itinerary. Still, Mattis, in his confirmation hearing this month, described “the Pacific theater” as a priority and analysts expect new U.S. military spending under Trump’s administration would strengthen America’s military presence in Asia over time. Topping U.S. concerns in the region are North Korea’s nuclear weapons and missile programs and China’s military moves in the South China Sea. Tension with Beijing escalated this week when Trump’s White House vowed to defend “international territories” in the strategic waterway. China responded by saying it had “irrefutable” sovereignty over disputed islands there. Mattis, in his Senate testimony, also voiced concern about North Korea, describing Pyongyang’s activities as a “serious threat” that required U.S. attention.    There are about 28,500 U.S. troops based in South Korea helping to defend the country against North Korea, which has technically remained in a state of war with the South since the 1950-53 Korean conflict. South Korea and the United States say the upcoming deployment of the Terminal High Altitude Area Defense (THAAD) system will better help to protect Seoul against North Korea’s nuclear and ballistic capabilities. But China says THAAD’s powerful radar could penetrate its own territory, prompting calls from some South Korean opposition leaders to delay or cancel its deployment.  