The United States urged North Korea Sunday to refrain from further "threatening" activity and engage in "serious and sustained" diplomacy, after Pyongyang fired several cruise missiles, the latest in a recent series of its weapons tests.

A State Department spokesperson made the call, noting that Pyongyang has increasingly engaged in "threatening" and "irresponsible" military activities, including missile launches.

Seoul's Joint Chiefs of Staff said that the North launched the missiles from waters off Sinpo, a city on its east coast, on Sunday morning, four days after it test-fired what it said were strategic cruise missiles, named Pulhwasal-3-31.

"We urge the DPRK to refrain from further threatening activity and call on Pyongyang to engage in serious and sustained diplomacy," the spokesperson said in response to a question from Yonhap News Agency. DPRK stands for the North's official name, the Democratic People's Republic of Korea.

"We remain committed to a diplomatic approach to the DPRK and call on the DPRK to engage in dialogue. Our commitments to the defense of the Republic of Korea and Japan remain ironclad," the official added.

The Republic of Korea is South Korea's official name.

In a separate comment, a Pentagon spokesperson said that the US will continue to work together with South Korea and Japan to tackle North Korean military threats.

"We are monitoring these activities. We have been very clear on the threat posed by the Democratic People's Republic of Korea military programs, and our commitment to the defense of the ROK, Japan, and our commitment to uphold regional peace and stability," the spokesperson said in response to a question from Yonhap News Agency.

"We will continue to work with both the ROK and Japanese governments to address the threats posed by the DPRK," the official added. (Yonhap)