A US think tank says North Korea has conducted a rocket engine combustion test in late April at a test facility of the Sohae satellite launching station in the country's northwest.

The Center for Strategic and International Studies, or CSIS, released on Monday findings from the analysis of satellite imagery of the launching station. The image was captured on April 29.

CSIS researchers say they confirmed burned vegetation and ground scarring near the engine test stand. They also say the imagery and several sources suggested that a test of a liquid propellant rocket engine had been carried out.

The South Korean military told reporters on Tuesday that it was aware of the combustion test at the launching facility.

The military said the North is making preparations for its launch of another military spy satellite, although there are no signs of an imminent launch.

North Korea announced last November that it put its first military reconnaissance satellite into orbit. Pyongyang has revealed a plan to launch three more satellites this year.