As NKorea aims nukes on US, SKorea and Japan feeling heat

WASHINGTON (AP) - 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 more unnerving for South Korea and Japan.

So much so that Washington is considering new ways to flex its nuclear muscle to defend its allies, while they ponder if they'll need nukes one day.

For decades, the U.S. has defended South Korea and Japan through an extended "nuclear umbrella."

The basic premise is that an attack on Japan or South Korea risked prompting an overwhelming and potentially nuclear American response.

But Pyongyang's emerging capability to strike the United States with a nuclear-tipped missile is upsetting all calculations.

And for America's allies, the most pressing security question might now be: Would Washington risk San Francisco for Seoul?