South Korea and Japan are the most important allies to the United States in the region and cooperation between the three countries is a key to regional peace and security, a Pentagon official said Tuesday.

John Supple, a spokesperson for the Department of Defense, also said the US will continue to seek to expand cooperation between the three countries.

"Trilateral cooperation among the United States, the Republic of Korea, and Japan is vital to the maintenance of regional peace, prosperity, and stability -- to include addressing the North Korea Nuclear, WMD, and ballistic missile threat and maintaining the rules-based international order," he told Yonhap News Agency in an email interview.

The remarks come after Tokyo expressed regret over Seoul's defense white paper that said military cooperation between the two has been hampered partly by Japan's territorial claim to the South Korea-controlled islets of Dokdo.

Supple said the US believes both Seoul and Tokyo share the importance of their three-way cooperation.

"In this vein, we will continue to seek every opportunity to expand our cooperation to meet our shared threats," he added.

The South Korean defense white paper also said North Korea continues to build up its nuclear capability, noting the number of its missile brigades under its strategic force command increased to 13 in 2020 from nine in 2018.

The Pentagon official refused to confirm such an assessment but said the North's continued development of weapons represent a threat to the US and its allies.

"North Korea's continued development of ballistic missiles and weapons of mass destruction represent a threat to US interests and the security of our allies and partners," Supple said.

"The United States remains committed to achieving the final, fully verified denuclearization of North Korea, as well as the dismantlement of its ballistic missile and weapons of mass destruction programs." (Yonhap)