Pyongyang is believed to have conducted three underground nuclear tests and is threatening a fourth, despite international condemnation and sanctions.
Last month, it announced it was restarting its main nuclear facility Yongbyon and that it was improving its nuclear weapons "in quality and quantity".
The North claims it has made a nuclear device small enough to fit into the warhead of a missile.
US officials, however, have cast doubt on this claim and experts say it is difficult to assess the progress North Korea has made on miniaturisation.
North and South Korea remain technically at war because the 1950-1953 war ended in an armistice, not a peace treaty.