TOKYO -- Increased activity at a North Korean nuclear site has once again caught the attention of analysts and renewed concerns about the complexities of denuclearization talks as President Donald Trump prepares for a summit with Kim Jong Un in the coming weeks. Satellite imagery taken last month suggests the North has begun preliminary testing of an experimental light water reactor and possibly brought another reactor online at its Yongbyon Nuclear Research Center.

Both could be used to produce the fissile materials needed for nuclear bombs.

The findings come at a particularly sensitive time. Trump and Kim are planning to meet by May, according to officials, and denuclearization will likely be the biggest topic on their agenda if they do meet. On Wednesday morning, President Trump tweeted twice about the looming meeting, saying he was looking forward to it and expressing hope that it could lead to the denuclearization of the Korean Peninsula.

For years and through many administrations, everyone said that peace and the denuclearization of the Korean Peninsula was not even a small possibility. Now there is a good chance that Kim Jong Un will do what is right for his people and for humanity. Look forward to our meeting! — Donald J. Trump (@realDonaldTrump) March 28, 2018

North Korea tested its biggest nuclear device to date last September. Pyongyang claims it was an H-bomb.

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While the North hasn't conducted any tests since, or test-launched any long-range missiles since Nov. 28, the heightened activity at the Yongbyon complex could be ominous.

According to an analysis in Jane's Intelligence Review published earlier this month, a testing program is now underway at the experimental reactor, which means it could become operational with "little warning later in 2018 or in 2019." It said the preliminary testing follows increased activity throughout 2017.

The reactor was completed five years ago and is primarily designed to generate electricity for civilian use. But it could also be used to produce plutonium or tritium.

The Jane's report cautioned that without international inspections it's impossible to tell for sure if it's being used to produce civilian electricity or weapons-grade material for bombs. The reactor has been linked to the local electricity grid and is believed to be potentially able to power a city of about 50,000 if operated at full capacity.

"Further monitoring of the type and cadence of activity at the site is likely to reveal clues about North Korea's ultimate intentions," the Jane's report concluded.

In a separate report posted on 38 North, a website that specializes in North Korea news and analysis, experts said they have detected activity at another reactor in the Yongbyon complex, which is located north of Pyongyang, that could be an even bigger concern.

Received message last night from XI JINPING of China that his meeting with KIM JONG UN went very well and that KIM looks forward to his meeting with me. In the meantime, and unfortunately, maximum sanctions and pressure must be maintained at all cost! — Donald J. Trump (@realDonaldTrump) March 28, 2018

Imagery of Yongbyon's 5-megawatt reactor suggests it was brought into operation very recently. The website said that would mean "North Korea has resumed production of plutonium presumably for its nuclear weapons program."

It said commercially available satellite imagery of the reactor revealed steam vapor plumes emanating from the generator hall, along with river ice melt near the reactor. Additionally, North Korean technicians might have stepped up efforts to conceal signs of the reactor's operations, "making monitoring efforts more difficult going forward."

The March 5 report added that a military tent camp has been set up at Yongbyon, possibly for troops doing construction or providing enhanced security.