Pyongyang watchdog 38 North reports tunnel excavation has been stepped up at the West Portal with “significant tunnelling” underway in the area.
Nine mining carts, vehicles and personnel have been seen at the site, this suggests Pyongyang could be readying the tunnel for further nuclear testing.
The spoil pile in the West Portal area greatly expanded along the south side.
The North Portal, where the last five nuclear tests were conducted, remains dormant with water draining from the portal entrance.
This follows extra activity noted at the West Portal site early last month.
North Korean intelligence had reported “construction had resumed at a fourth tunnel (West Portal), making it unable to be used “for a considerable amount of time”.
And the images show no new spoil has been added to the site, but presence and movement of equipment has significantly increased.
The West tunnel complex has been unused as yet, and no activity has been spotted in the area over the past several months.
Five of the past six nuclear tests were conducted at the North Portal, but no vehicles or equipment have been seen there since September.
A recent report by TV Asahi claimed hundreds of North Korean personnel died after the tunnels collapsed at the test site.
The three recent tremors could have caused damage to the tunnel networks but satellite images cannot confirm this.
There has been a possible thawing in tensions on the peninsula after officials from the two nations met face to face on Tuesday for the first time in more than two years in the border town of Panmunjom, known as “truce village,” in the Korean peninsula’s heavily fortified demilitarised zone.
But despite the olive branch from Kim Jong-un Alexander Vorontsov, a Russian academic and North Korea expert, said Pyongyang is convinced Washington is preparing to attack the hermit state.
Writing for website 38 North, Mr Vorontsov said he was told North Korea is so convinced war is about to about to break out soldiers have been “sleeping without removing their boots”.
He continued: “These officials feared that the US was already trying to shape the battlefield for a military operation against the North, and that South Koreans do not seem to have grasped the reality that the Trump administration is set on a course for preventive war.”
He said North Korea was not bluffing and actively preparing for war.
He said: “However, Pyongyang is extremely serious about this scenario and is not bluffing when it says that ‘only one question remains: when will war break out?’