South Korea’s Defense Ministry have warned that the North Koreans may be ready to conduct a fourth nuclear test, citing increased activity at Punggye-ri, the North’s primary test site.

Defense Ministry spokesman Kim Min Seok announced at a press briefing on Tuesday that South Korean military had been monitoring the activity of the site, where a nuclear test was conducted last year.

“We are thinking of possibilities that the North may stage a surprise nuclear test or just pretend to stage a nuclear test,” Kim said at the briefing.

Kim added that the North that the South Korean government believed