New satellite images have emerged of North Korean military bases appearing to show submarine activity at a test site just days after it threatened an attack on Guam.

The pictures reportedly show tarps and netting installed above the SINPO-class submarine to "obscure any activity taking place beneath them."

According to experts, the covers suggest North Korea could be preparing for test launches or be upgrading the submarines.

"Recent commercial satellite imagery reveals several developments suggesting that North Korea may be accelerating the development of the sea-based leg of its nuclear forces," military expert Joseph S. Bermudez Jr. from 38 North said.

View photos New satellite images have emerged of North Korean military bases appearing to show activity at a test site. Photo: Getty More

"Of particular interest in the imagery is that netting or tarps have been suspended above both the fore and aft decks of the SINPO-class submarine obscuring any activity taking place beneath them.

"This was last done prior to the July 9, 2016 test of the Pukguksong-1."

View photos According to experts, the covers suggest North Korea could be preparing for test launches. Photo: Getty More

View photos The pictures reportedly show tarps and netting installed above the SINPO-class submarine. Photo: Getty More

Pukguksong-1 was a submarine-launched ballistic missile that was successfully tested in August and travelled 500km towards Japan.

On Tuesday, Donald Trump warned the US would rain down “fire and fury” on Pyongyang if North Korea followed through with its threat to attack the US-held Pacific island of Guam.

View photos Reunification would raise problems for crime, migration and other social issues. Picture: Reuters More

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