Pyongyang had previously warned others not to shoot down a rocket it says it will launch between April 4 and April 8.
South Korean fighter pilots are 'ready to scramble' as North Korea prepares a missile launch.
The North claimed to have weaponized plutonium and said its nuclear status would not change. But analysts see bid for attention.
His trip, which ends at the Olympics, is intended to showcase achievements in the region.
The North's actions set the stage for another round in six-party talks, scheduled to take place next week in Beijing.
Bush lifts some sanctions in exchange for pariah nation's step toward nuclear cooperation.