Recent satellite imagery shows camouflage netting over the deck of North Korea's ballistic missile submarine in an indication that the communist nation is forging ahead with its SLBM development, a U.S. website monitoring the North said Tuesday.

The May 8 imagery of the Sinpo South Shipyard shows netting has been suspended over the Gorae-class submarine in an sign that the North is continuing to "actively pursue development of both a ballistic missile submarine and submarine-launched ballistic missiles (SLBM)," the website 38 North said.

"This netting was first seen in imagery of December 26, 2015 and on several occasions since. The purpose of this netting is to conceal ongoing activity. Netting for concealment purposes has also been observed at other submarine bases during the past five years," it said.

The imagery also showed that a submersible test stand barge, which was used previously to conduct SLBM tests, has been moved from its position along the northern secondary dock back to the main dock, suggesting that work is being undertaken to repair or modify the barge, 38 North said.

Also seen in the shipyard are two "mother ships" -- one 32 meters long and the other 40 meters-long -- which are used to transport intelligence agents and special operations troops on infiltration missions against South Korea and Japan, 38 North said.

North Korea's SLBM program has emerged as a key threat as submarines are difficult to track. Pyongyang has conducted a series of SLBM tests since last year, including one last month in which the missile flew about 30 kilometers.

U.S. experts said that despite the short distance, the test shows the North is making progress. (Yonhap)