Rare wailing air raid sirens and mobile phone alerts calling for evacuations rattled residents of the South Korean capital, Seoul, early on Wednesday after North Korea launched what it said was a satellite.

North Korea launched the rocket southward, South Korea's military said, prompting emergency alerts and evacuation warnings in parts of South Korea and Japan.

"I was so panicked. Nine-one-one lines were busy and the internet was slow," said Lee Juyeon, 33, a resident in the densely populated city of 9 million who has a 9-month-old child. "So without knowing what was really happening, I was about to head down to a basement wearing a wrap carrier with my baby."

The sirens started in Seoul at 6:32 a.m. (2132 GMT Tuesday) as the city issued a "Presidential Alert" asking citizens to prepare for a potential evacuation.

Then came a second mobile alert minutes later calling for an evacuation, which remained in place for at least 10 minutes until city said it had been sent in error.

Lee did not evacuate after seeing a television headline saying the alerts related to a North Korean space vehicle flying farther south, but she showed photographs of friends packing bags, readying to leave.

Although residents of Seoul are used to living in the shadow of threats from their nuclear-armed neighbour, an element of complacency has crept in among many in the city about the risks and how to respond.

The two countries are still technically at war seven decades after the Korean War ended in an armistice.

Some office workers in the Seoul's central district said they had considered during their commute how to respond to the alarm, such as by withdrawing cash or hoarding water.

"I understand it was a mistake, but even if it has to be simple, that alert message should have said what went on and where to go," Kim Jong-hyun, 48, said on his way to work after dropping off his child at school.

"Alerts" and "evacuation" were the most trending topics on Twitter in South Korea on Wednesday morning, with confused tweets scrambling to grasp what was going on or to find evacuation areas.

"Hey guys, given Twitter is still working, I guess it is not a war," one user using the handle @Kimisnim__ said.