North Korea had confirmed Tuesday that it planned to launch what it called "military reconnaissance satellite No. 1" before June 11, having already told Japan of its plans a day earlier.

Pyongyang fired "what it says is a space launch vehicle" southwards, Seoul's Joint Chiefs of Staff said, without giving further details.

Soon after, a text alert said: "Citizens, please prepare to evacuate and allow children and the elderly to evacuate first" as an air raid siren sounded in central Seoul.

Minutes after that, Seoul's interior ministry said the alert has been "incorrectly issued".

The Joint Chiefs of Staff said that the projectile flew over the Yellow Sea and did not affect the Seoul metropolitan area, Yonhap reported.

North Korea said Tuesday its new spy satellite would be "indispensable to tracking, monitoring... and coping with in advance in real time the dangerous military acts of the US and its vassal forces".

Criticising US-South Korea joint military exercises, including ongoing large-scale live-fire drills, a top North Korean military official said Pyongyang felt "the need to expand reconnaissance and information means and improve various defensive and offensive weapons", state media reported.

Pyongyang, which typically does not give advanced warning of missile launches, has been known to inform international bodies of purportedly peaceful satellite launch plans.

It told Japan Monday it would launch a rocket between May 31 and June 11.

Tokyo and Seoul strongly criticised the proposed launch, which they said would violate UN sanctions, which bar Pyongyang from any tests using ballistic missile technology.

Because long-range rockets and space launchers share the same technology, analysts say developing the ability to put a satellite in orbit would provide Pyongyang with cover for testing banned intercontinental ballistic missiles (ICBMs).

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

Japan briefly activated its missile alert warning system for the Okinawa region early Wednesday, lifting it after about 30 minutes.

- 'Price and pain' -

Since diplomacy collapsed in 2019, North Korea has doubled down on military development, conducting a string of banned weapons tests, including test-firing multiple ICBMs.

North Korean leader Kim Jong Un last year declared his country an "irreversible" nuclear power, and called for an "exponential" increase in weapons production, including tactical nukes.

Kim this month inspected the country's first military spy satellite as it was prepared for launch, and gave the green light for its "future action plan".

In 2021, Kim had identified the development of such satellites as a key defence project for the North Korean military.

South Korea's foreign ministry earlier this week condemned the launch plan, saying the "so-called 'satellite launch' is a serious violation of UN Security Council resolutions banning all launches using ballistic missile technology".

"If North Korea eventually goes ahead with the launch, it will have to bear the price and pain it deserves."