North Korea has confirmed it will launch a reconnaissance satellite in June, saying it is needed to monitor military movements of the United States and its partners in real time, state media reported on Tuesday, citing a senior defence official.

Japan said on Monday it had been informed by Pyongyang that a satellite launch could happen as early as this week, but Tokyo warned the North may in fact be planning a sanctions-defying ballistic missile test.

The North's state Korean Central News Agency cited Ri Pyong-chol, vice-chairman of the ruling party's central military commission, as saying the "military reconnaissance satellite No. 1" would be "launched in June".

That satellite, along with "various reconnaissance means due to be newly tested, are indispensable to tracking, monitoring... and coping with in advance in real time the dangerous military acts of the US and its vassal forces", the statement said.

Citing "reckless" acts by Washington and Seoul, Ri said North Korea felt "the need to expand reconnaissance and information means and improve various defensive and offensive weapons" in an effort to bolster military preparedness.

The official also accused the United States of conducting "hostile air espionage activities on the Korean peninsula and in its vicinity", according to the KCNA dispatch.

North Korea informed Japan it would launch a rocket between May 31 and June 11, identifying waters near the Yellow Sea, the East China Sea and to the east of Luzon Island in the Philippines as warning areas, said a Japanese coast guard spokesman.

Such zones are usually designated for falling debris or rocket stages.

Japanese Prime Minister Fumio Kishida told officials to gather intelligence "on North Korea's notification about the launch of a ballistic missile that it describes as a satellite", his office said in a tweet.

"Even if it's described as a satellite, a launch using ballistic missile technology would be a violation of United Nations Security Council resolutions" and would threaten people's safety, Kishida told reporters.

In 2012 and 2016, Pyongyang tested ballistic missiles that it called satellite launches. Both flew over Japan's southern Okinawa region. (AFP)