The possibility of North Korea launching a long range missile is causing jitters.
China says its concerned about regional peace and stability.
Pyongyang now claims U.S. spy planes are intruding and Japan has sent ships and planes to monitor for a possible launch.
The missile is believed to have enough range to reach the western United States.
Japanese media report Tokyo has dispatched a destroyer with advanced reconnaissance equipment and an attack aircraft loaded with radar-jamming electronics.
A ranking Japanese foreign ministry official is downplaying the possibility of the communist nation being capable of loading a nuclear warhead atop its rockets.
The official says there's "no indication" that North Korea has the technology to make an atomic weapon small enough.
The U.S. has rejected a North Korean offer for talks about the missile test.