North Korea has said a mishap happened as it wanted to send up its first space satellite, causing it to crash into the sea.

The rocket launch prompted Japan to warn residents in Okinawa but it later reported there was no danger of its territory being hit.

North Korea declared earlier it planned to launch a satellite by 11 June to monitor US military activities.

It now says it will attempt a second launch as soon as possible.

Japanese Top state leader Fumio Kishida said North Korea seemed to have fired a ballistic missile and that the government was analyzing the details.

He added that there were as of now no reports of harm following the launch. Japan said previously it was ready to shoot down anything that threatened its territory.

There was chaos and confusion in Seoul, the South Korean capital, on Wednesday morning local time, as people awoke to the sound of an air raid siren and an emergency message telling them to prepare for an evacuation - only to be told 20 minutes later it had been sent in error.

The stakes are high on the Korean Peninsula, where tensions have existed between the two countries for 70 years, and this false alarm could seriously damage people's trust in the alert system.

North Korea poses a threat to South Korea, and if there is an alert in the future one question being asked is whether it will be taken seriously, or brushed off as another mistake.

South Korea's military said the projectile might have broken up in mid-air or crashed after it vanished from radar early, adding that analysis was being conducted, Yonhap news agency reports.

On Tuesday, Ri Pyong Chol, vice chairman of North Korea's ruling party's central military commission, announced the launch plan, saying it was in response to "reckless military acts" by the US and South Korea.

He accused the countries of "openly revealing their reckless ambition for aggression".

Before Wednesday's launch, the US state department said any North Korean launch that used ballistic missile technology would violate multiple UN Security Council resolutions.

South Korea's foreign ministry also condemned the launch plan earlier this week, calling it a "serious violation" of 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," it said.

North Korean leader Kim Jong-un has identified the development of military satellites as a key component of his country's defense.

Leif-Eric Easley, a professor at Ewha University in Seoul, said the North Korean government "likely sees itself in a space race", and that whether or not its current satellite mission is a success it "can be expected to issue political propaganda about its space capabilities".