US President Donald Trump's defence secretary has told South Korea 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.
Mr 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 on Thursday.
Mr Hwang, who is serving as acting president after President Park Geun-hye was impeached over a corruption scandal, called for increasing pressure on Pyongyang.
North Korea conducted more than 20 missile tests last year, as well as two nuclear tests, in defiance of UN 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 US 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 January 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.
Mr Mattis' trip to the region, which will include a stop in Japan, is the first foreign trip by any of Trump's cabinet secretaries.
US officials have said the trip is meant to reaffirm ties with South Korea and Japan, US allies hosting nearly 80,000 American troops, and the importance of the region overall.
Both South Korea and the United States on Thursday recommitted to plans to deploy a US missile defence system, known as Terminal High Altitude Area Defense (THAAD), in South Korea later this year.
China, however, has objected to THAAD, saying it will destabilise the regional security balance, leading to calls from some South Korean opposition leaders to delay or cancel it.
Mr Mattis, without citing China explicitly, said "no other nation" needed to be concerned about THAAD.
"Were it not for the provocative behaviour of North Korea, we would have no need for THAAD out here," Mr Mattis said.