Feb. 20 (UPI) -- A U.S. spy plane flew over South Korea on Thursday amid concerns in the United States the North could be restarting operations at Yongbyon nuclear center.
Aircraft Spot, an aviation tracker, published data of the movements of the RC-135W, or Rivet Joint, on Thursday. According to the tracker, the spy plane flew at an altitude of 31,000 feet in South Korean air space.
Flight time was not provided. The Rivet Joint, designed to gather telemetry and other electronic intelligence data before missile launches, has been detected in South Korean airspace in recent months.
The aircraft appeared on Jan. 6, 7, 8, 21, according to Yonhap news agency. On Tuesday, a U.S. E-8C surveillance aircraft flew across South Korean airspace. In December, Aircraft Spot tweeted data of the movements of the Rivet Joint when it flew at an altitude of 31,000 feet near North Korea.
In 2019, North Korea had warned the United States to abide by a unilateral "year-end deadline," a reference to its demands for sanctions relief ahead of more substantive steps toward denuclearization.
In his New Year address, Kim Jong Un had vowed to introduce a "new strategic weapon," raising concerns in South Korea the regime could be pursuing the development of multiple independently targetable re-entry vehicles.
North Korea has mostly closed its border with China and Russia, but may be pursuing low levels of weapons activity.
According to U.S. analysts writing for 38 North, the regime's Yongbyon Nuclear Scientific Research Center is showing "minor activity."
"There are four railroad flatcars present, each configured to carry four to five cylindrical containers," U.S. analysts said last week.
"Their location and the lack of indicators at the Radiochemical Laboratory suggests that the rail cars are carrying nonradioactive materials, likely chemicals related to uranium conversion operations at the Radioisotope Production Plant."