North Korea's recent missile launches show the "seriousness of threat" posed to the United States and South Korea, a Pentagon official said Monday, amid a conciliatory gesture from Pyongyang after a series of weapons tests.

Siddharth Mohandas, US deputy assistant secretary of defense for East Asia, made the remark at the start of the biannual 20th Korea-US Integrated Defense Dialogue (KIDD) in Seoul to discuss regional security situations and pending alliance issues.

"North Korea's recent missile launches remind us of the seriousness of the threat we face together," Mohandas said, calling the alliance the "linchpin" of peace and security in the region against such challenges.

The remarks came two days after North Korean leader Kim Jong-un's sister, Kim Yo-jong, said the North could declare a formal end to the 1950-53 Korean War, as suggested by South Korea, and even discuss an inter-Korean summit if Seoul drops what she called a "hostile policy" against Pyongyang.

Kim's statement came amid concerns over the North's recent cruise and ballistic missile launches, and signs of the reclusive regime reactivating a key nuclear reactor at its mainstay Yongbyon complex.

Earlier in the day, US reconnaissance aircraft, including a Global Hawk, were spotted flying over the Korean Peninsula in an apparent move to track North Korea's activities following the missile launches and Kim's statement.

Referring to the recent talks of a formal end to the Korean War, Deputy Defense Minister Kim Man-ki said, "It is time a more close coordination between the South Korea-US alliance is required."

During the two-day meeting, the two sides plan to discuss "major pending security issues," such as the assessment of the security situation on the Korean Peninsula and the policy coordination on North Korea, according to the defense ministry.

Also on the table during this week's meeting will be the conditions-based transition of wartime operational control (OPCON) of South Korean troops from Washington to Seoul and ways to strengthen defense cooperation between the two countries, the ministry said.

Launched in 2011, KIDD is a comprehensive defense forum between Seoul and Washington that usually meets twice a year. The last session took place in Washington in May. (Yonhap)