"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."

Nuclear tests

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.

"The aggressive acts of war on the part of the United States are getting increasingly reckless," the official said.

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.

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.