North Korea has said that its attempt to launch the country's first spy satellite has failed.

In a statement published on state media on Wednesday, North Korea said a rocket carrying the spy satellite crashed into waters off the Korean Peninsula's western coast after it lost thrust following the separation of its first and second stages.

It said scientists were examining the cause of the failure.

The rocket was launched about 6:30 am local time from the North\'s northwestern Tongchang-ri area, where the country's main space launch center is located, the Joint Chiefs of Staff said in a statement.

South Korea's military earlier said the North Korean rocket had "an abnormal flight" before it fell in the waters.

It released images of part of a spy satellite that crashed soon after launch due to a rocket failure, with Seoul's military finding and salvaging the debris.

The images show a large barrel-like metal structure with some thin pipes and wires at the bottom, which South Korea's military said it salvaged in waters 200 kilometres west of Eocheong Island.

Earlier North Korea's launch prompting city officials in Seoul to issue a warning to citizens to prepare to evacuate.

Japan also issued and later withdrew evacuation alerts.

North Korea had said it will launch its first military reconnaissance satellite between May 31 and June 11 to boost monitoring of US activities.

In data provided to international authorities, North Korea said the launch would carry the rocket south, with various stages and other debris expected to fall over the Yellow Sea and into the Pacific Ocean.

The North's Kim Jong-un has made developing such a satellite a top military priority for his country, and has personally inspected it, approving its future launch.

North Korea does not have a functioning satellite in space, experts say.

Since 1998, Pyongyang has launched five satellites, three of which failed immediately and two of which appeared to have been put into orbit — but signals from them have never been independently detected, indicating they may have malfunctioned.

The most recent satellite launch was in 2016. The following year, Pyongyang successfully test-fired its first ICBM.

"Satellites launched by North Korea in the past were effectively ICBM tests disguised as normal satellites," An Chan-il, a defector-turned-researcher who runs the World Institute for North Korea Studies, told the AFP news agency before the launch.

"The upcoming launch is being advertised by Pyongyang as a state-of-the-art satellite," he said, adding it showed the North was eager to enter "the so-called 'military space era' before Seoul does".