How far is North Korea from being able to hit the U.S. with a nuke?

North Korea is shaking its fist at the world yet again this week.

Pyongyang is angry at the United Nations for toughening sanctions as punishment for the Hermit Kingdom's launching a long-range missile to carry a satellite into orbit in December. And on Friday, North Korea threatened to attack its democratic neighbor, South Korea, if it goes along with the measures approved by the U.N. Security Council. Also this week, North Korea vowed to test a nuclear device and fire more test missiles toward the U.S., which it called the "sworn enemy" of the Korean people. Such saber rattling is nothing new, but now even longtime ally China is threatening to cut back its aid if North Korea's isolated communist regime goes through with new nuclear tests. How seriously should Americans take Pyongyang's threats to fire a nuke our way? Here's what you should know:

Has North Korea already proved it has nuclear bombs?

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Yes, North Korea has conducted two underground nuclear tests, in 2006 and 2009. In both cases, it had just been hit with new U.N. sanctions after launching long-range rockets. North Korea says it has the right to build nuclear weapons to defend itself against the U.S. Nuclear scientist Siegfried Hecker, who visited North Korean nuclear facilities in 2010, says the country has enough weaponized plutonium to make four to eight bombs. In 2009, North Korean leaders announced they were also working on enriching their own uranium, which would give them another source of material to make nuclear weapons.

Could it fire them at the U.S.?

After North Korea shot its satellite into orbit, U.S. Defense Secretary Leon Panetta told American troops that the Unha rocket North Korea used was "an intercontinental ballistic missile, for God's sakes. That means they have the capability to strike the United States." Panetta's predecessor, Robert Gates, warned that North Korea would have missiles capable of hitting the continental U.S. as early as 2015. An Unha rocket could, in theory, deliver a payload the size of a nuclear warhead as far as Alaska, Hawaii, and even possibly part of the Lower 48. But North Korea still has a long way to go before it could launch such a strike.

Why isn't North Korea ready?

The biggest obstacle is coming up with a way to make a nuclear warhead small enough to fit on the tip of a missile. "Miniaturization is the big [challenge] — can you make a bomb small enough?" says Jeffrey Lewis, a nuclear policy expert at the Monterey Institute of International Studies. "It's related to re-entry [from orbit] in the sense that you have to put shielding to soak up the heat or burn off the heat. So the smaller you can make the warhead the more space you've got for re-entry shielding."

So the threat is overblown?

Well, there are plenty of things preventing North Korea from striking the U.S. — and making smaller nukes is just one. The isolated communist regime doesn't have enough money to feed its own people, so sustaining the kind of research it needs is tough. Also, sanctions are making it harder to bring in weapon parts from abroad. Plus, despite some successful tests, North Korea still lacks a reliable long-range missile — an Unha tested last April broke apart over the Yellow Sea. Nevertheless, there's still the danger that Pyongyang could truck a big nuke over the border into South Korea, and even if half of its long-range missiles splinter apart in the air, the other half would present a very real danger, nuclear experts say.

Sources: Associated Press, Christian Science Monitor, Globe and Mail, Los Angeles Times, New York Times