Satellite images showed increased activity at the Punggye-ri site, with mining carts and personnel frequently visible, and excavation waste piles growing, the respected 38North website said yesterday.
The underground base, which is carved into a mountain in the north-east of the hermit kingdom, has been the site of North Korea's last five nuclear tests.
The report continues: "On December 28, there were also a large number of personnel (100 to 200) observed in seven different formations whose goal is unknown in the Southern Support Area".
"Throughout December 2017, mining carts and personnel were consistently present around the West Portal and there was significant expansion of the spoil pile", reports 38 North.
38North experts believe that the spurt in activities at the Punggye-ri site indicates Pyongyang's continued efforts to keep the site ready for more nuclear tests.
Based on a trip to Pyongyang, Vorontsov said the atmosphere there was unstable and that the soldiers were "long been sleeping without removing their boots".
News of a tunnel reportedly crumbling at the nuclear site in October - likely because of the country's sixth and most powerful nuclear test in September, which potentially killed 200 people - sparked a lot of speculation about the site's future.
"These [North Korea] officials feared that the USA 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: "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?'"
Meanwhile, President Donald Trump boasted about having good relations with North Korean leader Kim Jon Un in an interview with the Wall Street Journal on Thursday. "I think you people are surprised".
But North Korea made no promises on its weapons programme at the talks, and lashed out when asked about denuclearisation by South Korean journalists, saying it was not up for discussion.
The talks resulted in the decision that North Korea would participate in the Winter Olympics in Pyeongchang, South Korea, in February.