BEIJING—The damage to the nuclear test site that North Korean leader Kim Jong Un has promised to shut down is more extensive than earlier assessments showed, according to a new study by a team of international scientists.

Space-based radar showed that after the initial impact of North Korea’s latest nuclear test in September last year, a much larger part of the Punggye-ri test site caved in over the following hours and days, according to a study published in Science magazine on Thursday. The study was conducted by researchers...