South Korea and Japan feel heat as North Korea aims nukes at US

As worrying as North Korea's nuclear advance is for America, the increasingly realistic threat of an atomic warhead striking a U.S. city might be even more unnerving for South Korea and Japan.

So much so that the United States is considering new ways to flex its nuclear muscle to defend its vulnerable allies as they ponder if they'll one day need atomic arsenals of their own.

For decades, the United States has defended South Korea and Japan, the nations most directly threatened by the North's missiles and massive conventional forces, through an extended ``nuclear umbrella.''

The basic premise is that an attack on either ally risked a devastating American response.

Pyongyang's emerging capabilities are upsetting all calculations.