North Korea has vowed to shoot down any Japanese planes that violate its airspace, accusing Tokyo of spying near one of its missile launch sites.

The military "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 North] even 0.001 mm," the air force said in a statement on Saturday.

The statement, carried by the country's official Korean Central News Agency, said Japan's E-767 surveillance aircraft conducted aerial espionage near the Musudan-ri missile site on its northeast coast on Wednesday and on Thursday.

North Korea has designated a no-sail zone off its eastern coast from June 25 to July 10 for military drills, raising concerns that it might test-fire short or mid-range missiles in the coming days, in violation of a UN resolution.

An official from Japan's defence ministry said the country's planes regularly gather information on North Korea but declined to comment on the types of planes used or the locations monitored.

He spoke on condition of anonymity, citing government policy.

Rare threat

The threat against alleged Japanese aerial espionage is rare, though North Korea has regularly complained of US spy missions in its airspace.

Japan is very sensitive to North Korea's missile programmes, as its islands lie within easy range.

In 1998, a North Korean missile flew over Japan's main island. Tokyo has since spent billions of dollars on developing a missile shield with the United States and has launched a series of spy satellites primarily to watch developments in North Korea.

But in April, another rocket flew over Japan's main island, drawing a strong protest from Tokyo.

Pyongyang claims it put a satellite into orbit, while the US and its allies say it was really a test of the country's long-range missile technology.