A US research institute said on Monday that North Korea tested a long-range rocket engine last month, citing new satellite images.

A US research institute said on Monday that North Korea tested a long-range rocket engine last month, citing new satellite images.

The US think tank could not identify the exact engine type. However possibilities included the second stage of the Unha-3 Space Launch Vehicle or the second or third stage engine of a much larger rocket under development, the US-Korea Institute at Johns Hopkins University posted on its 38 North website.

Analysis of before and after satellite photos indicated the test had taken place sometime between August 25 and 30 at the North's Sohae satellite launching station, the post said.

Analysis of before and after satellite photos indicated the test had taken place sometime between August 25 and 30 at the North's Sohae satellite launching station, the post said.

Sohae was the base for the successful launch of the Unha-3 rocket in December. The UN Security Council tightened sanctions against North Korea after the launch -- and then again after the North's missile test in February.