A US research group says satellite imagery of a rocket launching site in North Korea indicates that the country may be preparing to test a new engine.

The research group 38 North released on Saturday the result of its analysis on the images of the Sohae satellite launching station in Tongchang-ri taken the day before.

The group says a "retractable environmental shelter has been moved forward onto the ramp which connects the apron to the test stand" at an engine combustion test site, and "four to five vehicles have arrived on the apron."

The group also says, "During periods between tests, the shelter is typically parked next to the stand," adding that these movements suggest that North Korea could be preparing to conduct a test on a new liquid-fuel engine.

Pyongyang has admitted that the launch of its first military spy satellite on May 31 ended in failure.

It said the rocket carrying the satellite crashed into the sea after it lost thrust due to a problem with the second-stage engine.

It said the rocket was launched from the Sohae launching site.

Japan, the United States, South Korea and other countries remain on high alert, as North Korea says it will conduct a second launch as soon as possible.