It seems that the defense community is almost solely focused on whether or not North Korea can miniaturized their nuclear weapons enough to mount them on a long range missile. North Korea claims they have, but I'm hesitant to give any credence to North Korea's claims, yet at the same time the claim could be correct.

http://www.bbc.com/news/world-asia-35760797

North Korea 'has miniature nuclear warhead', says Kim Jong-un

State media published images showing the North's leader standing next to what it said was a miniaturised weapon.

The claim is impossible to verify from the images alone and experts have long cast doubt on such assertions.
Even if North Korea has not yet mastered the technology to miniaturize a nuclear weapon sufficiently to mount it on a missile, that doesn't mean they cannot deliver a nuclear weapon to the U.S.. During the Cold War we had backpack nukes, which would do a hell of a lot of damage, even if the amount of damage fell well short of a large nuclear weapon. I doubt North Korea has it, but regardless it indicates there are other ways to deliver a weapon. For example, even a large, crude nuclear weapon could theoretically be transported concealed in a ship and set off in a port of a large city.

http://www.pbs.org/wgbh/pages/frontl.../comments.html

Do "backpack" nuclear weapons exist?

Yes, small atomic charges exist. They are very small. Several dozen kilos, thirty kilos, forty kilos. I spoke with people that made them, I saw them. The American specimens can be seen on the Internet, they can be seen on photographs, they can even be seen in the movies. I have never seen Russian analogies, I have only seen American ones, but Russian ones do exist, because I spoke with people who made them, and I believe these people, these people knew what they were talking about.

How powerful are they?

Their power is about one kiloton, possibly less, but a powerful charge. You cannot destroy Moscow or London, but the Kremlin, you can destroy ... Capitol Hill can be wiped out by such a bomb. ...
http://www.homelandsecuritynewswire....port-container

The consequences of one nuclear bomb smuggled into a port in a container

A RAND study says that a 10-kiloton nuclear explosion at the Port of Long Beach could kill 60,000 people instantly, expose 150,000 more to hazardous radiation, and cause ten times more economic loss than the 9/11 terrorist attacks

A RAND Corporation study concluded that a nuclear explosion at the Port of Long Beach, California, could kill 60,000 people instantly, expose 150,000 more to hazardous radiation, and cause ten times more economic loss than the 9/11 terrorist attacks. The Santa Monica, California-based think-tank’s study examines the human casualties and infrastructure effects of terrorists detonating a 10-kiloton nuclear bomb in a shipping container after being unloaded onto a pier in Long Beach, which shares a waterway with the Port of Los Angeles.