"The activity during the past six weeks is suggestive of the final preparations for a test," 38 North analyst Joseph Bermudez told CNN.
He and other analysts pore over commercial satellite imagery of the testing site, looking for signs of activity similar to that prior to other tests.
Bermudez and his colleague Jack Liu correctly predicted the last nuclear test
, in September 2016. Since late February they have been warning of increased activity at the site
, particularly new equipment arriving, activity at the command center, and excavation and water pumping from entrances to the underground testing site.
"We watch what changes from image to image," he said. "We take the current image and look back several months and compare it to what was happening then."
After an uptick in recent months, recent satellite images show excavation work and water pumping appears to have stopped, Liu and Bermudez said, indicating to them that a test may be imminent.
Speaking at an event in Tokyo Thursday, Yukiya Amano, director general of the International Atomic Energy Agency, said "North Korea's nuclear program is showing steady progress."
North Korea has conducted five nuclear tests in total, three of them under current leader Kim Jong Un, who has significantly ramped up the country's program.
"North Korea believes the only way to deter the US from attacking them and maintaining the power of the Kim regime is by the possession of nuclear weapons," Bermudez said.
North Korean officials told CNN in Pyongyang this week
that the decision by US President Donald Trump to bomb a Syrian airfield, as well as his dispatching a US navy carrier strike group to northern Asia
, showed the importance of the nuclear program.
"The aggressive acts of war on the part of the United States are getting increasingly reckless," the official said.
Trump has promised to "be effective" on North Korea, and called on China to play a role containing the regime's nuclear program
. Following a talk between him and Chinese President Xi Jinping Wednesday, Trump said China "wants to do the right thing."
Bermudez said the regime has "repeatedly stated that if Libya or Iraq had nuclear weapons the US would not have attacked them."
North Korea will mark the "Day of the Sun" on April 15, to mark the 105th anniversary of the birth of the country's founder Kim Il Sung. The date has typically been a key one for weapons tests and other displays of strength.
However, none of the five previous nuclear tests have been conducted around the anniversary, and while Bermudez said the evidence suggests there is "probably a sixth coming up," ultimately preparations did not mean much without a final say so from the country's supreme leader.
"It really depends on one person and that's Kim Jong Un," he said. "No one else is going to make the decision."
Day of the Sun
Previous years have seen grand military parades, mass celebrations and weapons tests conducted on the "Day of the Sun."
"It's a demonstration of national pride," Bermudez said, adding that satellite imagery appeared to show thousands of people and hundreds of vehicles practicing this week for the event.
Ahead of this year's anniversary, Kim presided over a drill by special operations units, helicopter gunships and other units, according to state news agency KCNA.
"(Kim said) those combatants carrying out their duties independently and proactively were reminiscent of fierce tigers," KCNA reported Thursday, adding that the timing of the drill was "significant" given Kim Il Sung's upcoming memorial.
Often the "Day of the Sun" is a key time for Pyongyang to show off its latest armaments. In 2012
, months after Kim Jong Un became leader following his father's death, North Korea unveiled what it claimed was its first intercontinental ballistic missile (ICBM) in a huge military parade.
Last year, South Korean officials said the North tested an intermediate-range Musudan missile
, but it failed.
The test was not publicized inside North Korea. Weapons expert Markus Schiller told CNN last week the Musudan is likely North Korea's most domestically developed missile, unlike others that are based on Soviet or Chinese designs, making it a key point of pride for the regime.
In terms of national pride though, nothing comes close to the nuclear program, and Pyongyang may decide that the best way to honor its late leader -- and defy pressure from the US and China -- is with a sixth nuclear test.