By Phil Stewart and Jack Kim
SEOUL (Reuters) - U.S. President Donald Trump's defenses secretary told South Korea on Thursday the two allies would stand "shoulder-to-shoulder" to face the threat from North Korea, in a message of reassurance after Trump questioned aspects of the alliance in his campaign.
Jim Mattis' two-day visit comes amid concern that the North may be readying to test a new ballistic missile, in what could be an early challenge for Trump's administration.
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Mattis, in his first trip abroad as Pentagon chief, vowed to strengthen ties in talks with South Korean Prime Minister and acting President Hwang Kyo-ahn.
"Right now we have to address the reality of the threat that your country and my country faces and we intend to be shoulder-to-shoulder with you as we face this together," Mattis said at talks with Hwang in Seoul.
Hwang, who is serving as acting president after President Park Geun-hye was impeached over a corruption scandal, called for increasing pressure on Pyongyang, including by strengthening sanctions and pressing forward with a strong, united defenses.
"South Korea and the United States must try to extract a change in North Korea's strategic calculus by deterring the North's aggression," Hwang said in a statement.
North Korea, which regularly threatens to destroy South Korea and its main ally, the United States, conducted more than 20 missile tests last year, as well as two nuclear tests, in defiance of U.N. resolutions and sanctions.
The North appears to have also restarted operation of a reactor at its main Yongbyon nuclear facility that produces plutonium that can be used for its nuclear weapons program, according to a U.S. think-tank, 38 North.
In his New Year's speech, North Korean leader Kim Jong Un said his nation was close to test launching an intercontinental ballistic missile (ICBM).
Trump on Jan. 2 tweeted, "It won't happen!" about North Korea's pursuit of a nuclear-tipped ICBM, although his precise meaning was unclear. The Pentagon has said it would not necessarily strike a test-launched ICBM if it did not pose a threat.
Mattis' trip to the region, which will include a stop in Japan, is the first foreign trip by any of Trump's cabinet secretaries.
U.S. officials have said the trip is meant to reaffirm ties with South Korea and Japan, U.S. allies hosting nearly 80,000 American troops, and the importance of the region overall.
That U.S. reaffirmation could be critical after Trump appeared to question the cost of such alliances during the election campaign. Trump also jolted the region by pulling Washington out of an Asia-Pacific trade deal that Japan had championed.
Both South Korea and the United States on Thursday recommitted to plans to deploy a U.S. missile defenses system, known as Terminal High Altitude Area Defense (THAAD), in South Korea later this year.
"THAAD is for defenses of our ally's people, of our troops who are committed to their defenses," Mattis told reporters shortly before landing in South Korea.
South Korea and the United States say the deployment of THAAD is designed to protect against North Korea's growing nuclear and ballistic capabilities, which have been advancing despite years of efforts by the international community.
China, however, has objected to THAAD, saying it will destabilize the regional security balance, leading to calls from some South Korean opposition leaders to delay or cancel it.
Mattis, without citing China explicitly, said "no other nation" needed to be concerned about THAAD.
"Were it not for the provocative behavior of North Korea, we would have no need for THAAD out here," Mattis said.
Mattis, who will hold more talks in Seoul on Friday before heading to Japan, suggested he was coming into a region with an open mind about what might be working - and what might not be - in the strategy to deal with Pyongyang.
"Strategies are games of give and take and I have to see their (South Korea's) view of it," Mattis said.
"Have we maintained what passes for peace so far? To a degree we have. But we're not trying to just look at today. We're looking at today and we're looking at tomorrow. And I need to see where they view this going right now."
Once fully developed, a North Korean ICBM could threaten the continental United States, which is about 9,000 km (5,500 miles) from North Korea. ICBMs have a minimum range of about 5,500 km (3,400 miles), but some are designed to travel 10,000 km (6,200 miles) or more.
Former U.S. officials and other experts have said the United States essentially has two options when it comes to trying to curb North Korea's fast-expanding nuclear and missile programs - negotiate or take military action.
Neither path offers certain success and the military option is fraught with huge dangers, especially for Japan and South Korea, U.S. allies in close proximity to North Korea.
(Editing by Nick Macfie, Robert Birsel)