On Jan. 19, North Korea said it would bolster its defences against the United States and consider resuming "all temporarily suspended activities", according to state news agency KCNA, an apparent reference to the self-imposed moratorium.
New construction has also been spotted at North Korea's only known nuclear test site, which was shuttered in 2018.
The looming prospect of possible nuclear tests, more joint U.S.-South Korea military drills, and the new conservative South Korean president mean "all conditions are present for a tit-for-tat chain reaction of escalatory steps", said Chad O'Carroll, CEO of Korea Risk Group, which monitors North Korea.