Reacting to the report President Trump stated that future threats would be "met with fire and fury and frankly power, the likes of which this world has never seen before." In response North Korea announced that it was examining an operational plan to strike areas around the U.S. territory of Guam in the Pacific, including the Andersen Air Force Base. " and Naval Base Guam which includes nuclear attack submarine (SSN) Squadron 15.
Even if North Korea hasn't yet miniaturised warheads small enough to fit on a missile or torpedo, North Korea could try to deliver larger nuclear devices (maybe 2+ tons), especially to South Korea and/or Japan by other means, including North Korean:
- mini to medium submarines carrying fitted nuclear demolition charges. The subs would need to be
manned by a crew prepared to die or escape by diver delivery/propulsion vehicle (which allow
divers to "swim" away faster).
- nuclear devices on "civilian cargo ships" or "trawlers" that may have been at sea for days-weeks.
Escape the explosion using fast rigid dingies.
- on aircraft: transport, "civilian" aircraft, or regular reconnaissance aircraft. Bomber aircraft may be
a bit obvious. A semi-suicidal crew would need to parachute or eject.
- a well disguised truck load?
- via a tunnel under North Korean-South Korean border.
All of these methods would be near suicidal for the delivery crew. I don't know how frequently North Korean troops or agents are prepared to carry out suicide missions, but there is a history of it from the 1996 Gangneung submarine incident.