"The physics of the event that we are talking about today seem to indicate a much larger event than the one from 2016 and before," Zerbo said.
The CNTBTO chief said that North Korea's tests had grown progressively more powerful since 2006 with each new trial.
According to him, the first registered event was at the "highest threshold" in terms of the organization's detection capability.
"And eight minutes later we had a subsequent event, we have several possibilities that are in line… and we'll be able to give you in the days or hours to come further details on this issue," Zerbo said.
Earlier in the day, Japanese Defense Minister Itsunori Onodera said that a nuclear weapon that was tested by North Korea on Sunday is preliminarily estimated to have a yield of up to 70 kilotonnes.
The Japanese government has condemned North Korea in the strongest terms through its embassy in Beijing and will coordinate its actions with the United States and South Korea to request the United Nations to convene an emergency Security Council meeting, Kono said as quoted by the NHK broadcaster.