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, an event condemned by the West as a disguised long-range ballistic missile test that violated UN resolutions.
The UN Security Council tightened sanctions against North Korea after the launch and then again after the North's missile test in February. Separate satellite images analysed by the US-Korea Institute last month showed that North Korea has embarked on a major new construction programme at Sohae.
The building work included what could be a possible new launch pad for testing mobile ballistic missiles.