The North's firing of submarine-launched cruise missiles is seen in this file photo carried by North Korea's official Korean Central News Agency, Jan. 29. Yonhap

The U.S. mission to the United Nations expressed "serious concern" Tuesday over North Korea's launches of what it claimed to be strategic cruise missiles this week, vowing to work with all members of the U.N. Security Council (UNSC) to address Pyongyang's threats.

The mission gave the comment to Yonhap News Agency amid speculation that Pyongyang has been engaging in the tests as the cruise missiles, unlike ballistic missiles, are not subject to UNSC resolutions despite their evolving threats.

"All launches by the DPRK are of serious concern, and the United States reiterates its condemnation of Pyongyang's WMD and ballistic missile launches programs," a spokesperson of the U.S. mission said. DPRK stands for the North's official name, the Democratic People's Republic of Korea.

"We have and will continue to engage with all members of the U.N. Security Council to address these ongoing threats to international peace and security," the official added.

The North conducted a drill to launch the Hwasal-2 strategic cruise missile Tuesday, two days after it fired what it claimed to be submarine-launched cruise missiles, named Pulhwasal-3-31, according to its official Korean Central News Agency.

Hwasal means "arrow" in Korean, and Pulhwasal means "fire arrow." (Yonhap)