A US-based research group says satellite imagery shows busy activity at a launching station in North Korea. It says this could indicate the North is preparing another launch after the failed attempt on Wednesday to put its first military reconnaissance satellite into space.

The group, 38 North, analyzed satellite imagery of the Sohae launching station in the country's northwest. The photos were taken about four hours after the failed launch.

The research group says one image shows a significant level of activity on the main launch pad that was used in the past. It is reportedly different from the new coastal pad used for Wednesday's launch. The group says there are over a dozen vehicles on the old launch pad, and a truck with a long trailer is parked nearby, possibly carrying rocket fuel.

It says the activity could indicate another launch is "forthcoming."

Japan, the US and South Korea remain on alert as they think the North is likely to fire a ballistic missile again in the guise of a satellite launch.

North Korean leader Kim Jong Un's sister, Kim Yo Jong, said in a statement on Thursday that Pyongyang will soon make a second attempt to put a reconnaissance satellite into orbit.

North Korea has informed Japan that it would launch a satellite sometime between Wednesday and June 11.

UN Security Council resolutions prohibit North Korea from conducting launches using ballistic missile technology.