Kune Y. Suh, a nuclear engineering professor at Seoul National University, said the latest test, if confirmed, appeared to be two to three times larger than earlier tests. Japan also sent four jets to test for radiation. China’s environment ministry began emergency radiation monitoring along its borders with North Korea in northeast China. Japanese Prime Minister Shinzo Abe told Friday that if North Korea had conducted another nuclear test it was “absolutely unacceptable.” North Korea has taken part in multiple weapon’s tests recently, in displays of force intended to demonstrate the country’s developing nuclear capabilities. International reactions are expected. North Korea has been under U.N. Security Council sanctions since it first tested an atomic device in 2006.