US Defense Secretary Jim Mattis departed for South Korea on Wednesday in his highly symbolic first overseas trip designed to underscore the US security commitment to the Asian ally in the face of a saber-rattling North Korea.

The trip, which also includes a stop in Japan later this week, comes as tensions are running high on the Korean Peninsula after North Korean leader Kim Jong-un said the country is close to test-firing an intercontinental ballistic missile apparently capable of reaching the US.

It is Mattis' first overseas visit since taking office less than two weeks ago. That underscores the seriousness and urgency with which the retired Marine Corps general takes the threats from North Korea, as well as the importance of the alliances with South Korea and Japan in dealing with the provocative regime.

More than anything else, the trip is aimed at reassuring the key allies unsettled by US President Donald Trump's campaign rhetoric accusing them of free-riding on US security commitments, though Trump has significantly toned it down since the election and has talked increasingly about the importance of alliances.

"Considering the fact that North Korea could test a long-range missile in the very near future, Secretary Mattis must find a way to make it clear to his counterparts in South Korea that Washington will not only honor its treaty commitments, but that there is no daylight between Washington and Seoul," said Harry Kazianis, director of defense studies at the Center for the National Interest, formerly known as the Nixon Center.

"Sending James Mattis certainly is a very clear signal that Washington is committed to South Korea's defense, now and in the future," he said.

In Seoul, Mattis will hold talks with South Korean Defense Minister Han Min-koo.

Earlier this week, the two spoke by phone, during which Mattis reaffirmed the "US commitment to defend the ROK and provide extended deterrence using the full range of US capabilities,"
according to the Pentagon.

Mattis also told Han that the US will "stand shoulder-to-shoulder with the ROK forces" and the two sides agreed to take steps to strengthen the alliance further to defend against the evolving North Korean threat," according to the Pentagon.

According to officials in Seoul, Mattis and Han also reaffirmed the planned deployment of the US THAAD missile defense system in South Korea this year to defend against North Korean missile threats. China has denounced the upcoming deployment as detrimental to its security, despite repeated assurances from Washington and Seoul that the system is purely defensive.

During his confirmation hearing last month, Mattis said the North's nuclear and missile development is "a serious threat and we've got to do something about it." Asked if "necessary force should be on the table" to deal with the threat, Mattis said, "I don't think we should take anything off the table."

He is a strong advocate of alliances.

In a message to Pentagon employees shortly after his confirmation, Mattis vowed to work closely with the State Department to strengthen our alliances, saying he recognizes "no nation is secure without friends." During the confirmation hearing, Mattis also said the US is "stronger when we uphold our treaty obligations."

Other topics that could come up during Mattis' talks with Han include defense cost-sharing.

Trump pledged on the campaign trail to have wealthy allies like South Korea and Japan shoulder more of the cost of American troops stationed in the countries, saying the US should prepared to withdraw the troops unless the allies agreed to pay more.

Trump has toned down the campaign rhetoric since the election.

Still, he spoke critically in his inauguration address of spending money to help defending allies, saying the US has "subsidized the armies of other countries" and "defended other nations' borders."

Mattis also said during his confirmation hearing that while the US fulfills its treaty obligations, he expects "our allies and partners to uphold their obligations as well." Incoming Secretary of State Rex Tillerson said in his confirmation hearing that the US "cannot look the other way at allies that do not meet their obligations." 

About 28,500 US troops are stationed in South Korea to deter North Korean aggression, a legacy of the 1950-53 Korean War, which ended in a truce, not a peace treaty. Seoul pays about half the costs, about $900 million a year, to help finance the troop presence.

Many agree that the troop presence is also in line with US interests in a region marked by China's rise. (Yonhap)