SEOUL, SOUTH KOREA - North Korea warned that any Japanese plane entering its airspace would be shot down for spying, as recent surveillance suggests Pyongyang may be preparing to fire more missiles.

"The air force of the Korean People's Army will not tolerate even a bit the aerial espionage by the warmongers of the Japanese aggression forces but mercilessly shoot down any plane intruding into the territorial air of the DPRK (North Korea) even 0.001 millimeter," the air force said in a statement.

It said a Japanese AWACS aircraft made a long shuttle flight into airspace between the cities of Wonsan and Musudan-ri on Friday morning.

Similar aerial espionage was committed the previous day, it said.

The North used Musudan-ri for its three previous long-range missile launches, in 1998, 2006 and on April 5 this year.

North Korea will likely fire short- or mid-range missiles in waters off its east coast from which it has banned shipping, a senior South Korean government official said Wednesday.

The North has warned foreign ships to stay clear of an extensive area for 16 days starting Thursday because of unspecified military exercises.

Yonhap news agency, quoting a government source, said the communist state would probably fire Scuds with a range of up to 500 kilometres or ground-to-ship missiles with a 160-kilometre range into the Sea of Japan, or East Sea.

Washington has said it is prepared for the possibility that the North could also fire a long-range missile towards Hawaii, perhaps on July 4, US Independence Day.