Joseph Bermudez is a specialist in North Korean defense and intelligence affairs. He posted the photographs on the authoritative '38 North' blog of the US-Korea Institute at Johns Hopkins University.

Bermudez said the pictures taken in May show similar preparations to those that took place before North Korea's last submarine launched missile test in August last year.

During that test the missile flew some 300 miles towards Japan, which leader Kim Jong-Un said at the time put the US mainland within striking range from a Pacific-based submarine.

Bermudez said, "Since the July report, netting or tarps have been suspended above both the fore and aft decks of the submarine obscuring any activity taking place beneath them."

Bermudez says activity on the experimental ballistic missile submarine at the Mayang-do navy shipyard and submarine base suggests "that the North may be preparing for a new series of 'at sea' test launches, has undertaken modifications or upgrades to the submarine's launch systems, or is developing a more advanced version of the Pukguksong-1."

Meanwhile, US-South Korean joint military exercises are due to begin later this month. The annual exercises, called Ulchi-Freedom Guardian, are expected to run from August 21 to 31.

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