The meeting was initiated by the United States and Japan, the source said.
Earlier in the day, North Korea announced a successful test of a hydrogen bomb in a special statement broadcasted by the central TV and South Korean TV channels.
According to the statement, Pyongyang claimed the reason for the test was the country's right for "self-defense," and promised not to use nuclear weapons if the international community respects its sovereignty.
North Korea conducted nuclear tests in 2006, 2009 and 2013.