The United States has reaffirmed its commitment to the defense of South Korea against threats from North Korea after Pyongyang tested a new strategic cruise missile on Sunday, the second in less than a week.

A U.S. Defense Department spokesperson said on Sunday that Washington has articulated its understanding of the threat posed by the regime’s military programs as well as the U.S.’ commitment to the defense of South Korea and Japan as well as the maintenance of regional peace and stability.

The spokesperson added that the U.S. is monitoring the North’s activities and will continue to work with both Seoul and Tokyo to address the threats posed by Pyongyang.

A State Department spokesperson said that North Korea has increasingly engaged in threatening and irresponsible military activities, including missile launches, and urged the North to engage in serious and sustained diplomacy.

South Korea’s Joint Chiefs of Staff said on Sunday that North Korea fired multiple cruise missiles in waters off the port city of Sinpo in South Hamgyong Province at around 8 a.m.

The launch came just four days after the first test-launch of a new strategic cruise missile named the Pulhwasal-3-31.