An object believed to be a satellite put into space by North Korea's recent rocket launch is on a near-circular orbit at an altitude of 494-588 kilometres passing through the north and south poles, a US expert has said.

Jonathan McDowell, a researcher at the Harvard-Smithsonian Center for Astrophysics, analysed data about the launch tracked by the North American Aerospace Defense Command and made available for astronomers and other experts.

The NORAD data also confirmed the orbit of two other objects believed to be the third stage of the rocket and its separation system.