OSAN AIRBASE -- U.S. Defense Secretary Ashton Carter arrived in Seoul on Thursday for a three-day visit aimed at further cementing the bilateral alliance against North Korea’s persistent nuclear and missile threats.

His visit came two days after Pyongyang test-fired two KN-06 surface-to-air missiles from Hwajin-ri, South Pyeongan Province, in an apparent show of force amid the prolonged deadlock in its relations with Seoul and Washington.

Carter’s first visit to Korea as the Pentagon chief followed his three-day visit to Japan where he portrayed the U.S-Japan alliance as the “bedrock” of America’s alliance system in the Asia-Pacific.

After touching down at the U.S.’ Osan Airbase south of Seoul, Carter was greeted by U.S. Ambassador to South Korea Mark Lippert, U.S. Forces Korea Commander Gen. Curtis Scaparrotti, U.S. 7th Air Force Commander Lt. Terrence O’Shaughnessy and South Korea’s Air Force Operations Commander Lt. Park Jae-bok.

Carter toured the 36th Fighter Squadron, a U.S. air force unit operating the F-16 fighter fleet, and then attended a town hall meeting with some 200 U.S. troops, where he appreciated their contribution to keeping the alliance and the U.S. strategic engagement in the region.

“You, by being as ready, skilled and formidable as you are, deter attack and keep the peace. I just want you to know that you are what I wake up for everyday. We don’t take it for granted,” Carter said during the town hall meeting.

Touching on Washington’s “rebalance” policy toward the Asia-Pacific, the secretary stressed that the U.S. would commit the new high-tech equipment in the region, which accounts for half of the world’s population and wealth.

“A lot of our newer capabilities that we are investing in are going to be flowing into this theater because of the importance we attach to it,” he said. “You will see a lot of new stuff showing up here first. And that is deliberate and that is appropriate.”

On Friday, Carter and his South Korean counterpart Han Min-koo will hold talks to discuss bilateral, regional and global security challenges and their cooperation in handling them.

On top of the agenda is North Korea’s escalating nuclear missile threats, a reason why observers expect them to discuss the U.S.’ potential deployment of the Terminal High-Altitude Area Defense system, a core element of America’s multilayered ballistic missile shield.

THAAD has emerged as a hot-button diplomatic issue as Beijing has repeatedly expressed opposition to it based on its belief that the deployment would hurt the regional security balance, escalate military tension and potentially spark an arms race.

Seoul officials say that the THAAD issue is not included in the official agenda. But many anticipate that the issue would be touched upon unofficially during the talks between Han and Carter.

This week, senior U.S. officials have played up North Korea’s nuclear missile threats, sparking speculation that Washington appears to be underscoring the need for THAAD in Korea and a stronger missile defense program elsewhere.

Aside from the North’s nuclear and missile threats, the two ministers are also to discuss developing follow-up measures to implement the “conditions-based” transition of wartime operational control.

They will also consult over the issue of crafting a set of documents to replace the Strategic Alliance 2015 -- detailing procedures for the OPCON transfer in 2015.

Last October, the allies agreed to a “conditions-based” transfer of wartime operational control in light of evolving North Korean threats, finalizing their decision to delay the transfer, previously scheduled for December 2015. They did not set a date for the transition, but the transfer is expected to occur in the early or mid-2020s.

After the talks between the two ministers, Han and Carter will visit the 2nd Fleet Command in Pyeongtaek, Gyeonggi Province, where the wreckage of the ill-fated corvette Cheonan is on display. Carter will be the first U.S. defense chief to visit the wreckage.

The Cheonan was sunken by a North Korean torpedo attack in March 2010, which killed 46 sailors. Pyongyang, however, denies responsibility for the attack, arguing that Seoul’s blame on Pyongyang was an “outright fabrication.”

Carter’s visit to Japan focused on highlighting the importance of the U.S.-Japan alliance for maintaining peace and stability in the Asia-Pacific. He touted the alliance as the bedrock of America’s alliance network, stressing joint efforts to bolster the bilateral security cooperation.

At the talks between Carter and Japan’s Defense Minister Gen Nakatani, the issue of revising the 1997 bilateral defense cooperation guidelines topped the agenda.

The revision work, expected to be completed in the first half of this year, will charter a future path for the defense collaboration to tackle new challenges including China’s assertiveness in maritime disputes, analysts say.

“The guidelines give us opportunities to have reflected on this alliance which has been the bedrock of our alliance system in this part of the world and a foundation of security in the entire region … give us the opportunity to extend its reach into new domains like space and cyber,” said Carter during a joint news conference in Tokyo.

“It really allows us to open up many new opportunities to strengthen the alliance in areas where that strengthening is needed because the security situation has changed, or in areas where we have new opportunities as a consequence of the guidelines.”

The revision of the guidelines is expected to contain a set of joint measures to cope with Chinese threats and other emerging scenarios. First adopted in 1978 to counter Soviet threats, the guidelines were last amended in 1997 to reflect post-Cold War security threats.

By Song Sang-ho (sshluck@heraldcorp.com)