Satellite imagery of a North Korean shipyard on Friday shows activity suggestive of preparations for a test of a medium-range submarine-launched ballistic missile, The Centre for Strategic and International Studies (CSIS) reported on Friday.

The US think-tank said the images it published on its website of North Korea's Sinpo shipyard showed several vessels within a secure boat basin, one of which resembled vessels previously used to tow a submersible test stand barge out to sea.

North Korea said last October it had successfully test-fired a Pukguksong-3, a new submarine-launched ballistic missile (SLBM), from the sea as part of efforts to contain external threats and bolster self-defence.

There was no immediate comment from the State Department or the Pentagon on the CSIS report.

Republic of Korea's military said the; PKS-3 tested last year flew 450 km (280 miles) and reached an altitude of 910 km (565 miles) and would have had a range of about 1,300 km (800 miles) on a standard trajectory.