American officials described the new “counterprovocation” plan as calling for an immediate but proportional “response in kind” — hitting the source of any North Korean attack with similar weapons. For example, if the North Koreans were to shell a South Korean island that had military installations, as has occurred in the past, the plan calls for the South to retaliate quickly with a barrage of artillery of similar intensity.

South Korea’s national security director said Sunday that the North this week might launch one of its new Musudan missiles, a modified version of a missile Russia used for decades aboard its submarines. If so, Pentagon officials said they would be ready to calculate its trajectory within seconds and try to shoot it down if it appeared headed toward impact in South Korea, Japan or Guam, an American territory. But they planned to do nothing if it were headed toward open water, even if it went over Japan, as one previous North Korean test did.

The officials doubted that the North’s new leader, Kim Jung-un, would risk aiming the missile at the United States or its allies.

Mr. Obama, officials say, has ruled out striking at the missiles while they are on their launchers — when they are easiest to destroy — unless there is evidence they are being fitted with nuclear warheads, which intelligence officials doubt North Korea yet possesses.