(CNN) New satellite imagery examined by Western experts suggests North Korea has begun preliminary testing of one of its nuclear reactors at the Yongbyon research facility. The disclosure comes as preparations get underway for the summit between North Korean leader Kim Jong Un and South Korean President Moon Jae-in next month -- and ahead of Kim's planned meeting with President Trump in May.

A report by intelligence analysts Jane's says the imagery indicates the experimental light water reactor, known as an ELWR, could become operational "with little warning" as early as later this year.

According to Jane's, an image from February 25 shows an emission rising from the reactor's stack that "implies testing of the machinery at the site." The stack is "intended to vent noncondensable gases from the reactor's primary circuit," Jane's says.

What is unclear at this stage is whether North Korea plans for the reactor to contribute to electricity generation or its weapons program.

Rob Munks, editor of Jane's Intelligence Review, says the light-water reactor "could be used for civilian electricity generation -- its stated purpose -- or diverted towards the nuclear program."

The reactor is linked to the power grid. Industry experts say that once operational, the ELWR would be able to produce about 25-30 megawatts, perhaps enough to power a town of some 50,000 inhabitants.

DigitalGlobe imagery showing emissions from the stack at the Yongbyon experimental light water reactor in February.

Munks said, "In theory, if the reactor comes online and if it were diverted towards plutonium and tritium production, it could enable North Korea to expand its stock." By just how much is unclear, he said. Tritium is the most important thermonuclear material for weapons.

Over the last year Jane's and other research groups have identified increased activity in several parts of the Yongbyon site, 40 miles (75 kilometers) north of Pyongyang. Analysts at Stanford University's Center for International Security and Cooperation observed the installation of power lines, a construction and dredging project to supply cooling water to the ELWR and movement of personnel and vehicles.

Construction of the ELWR was completed in 2013 and is optimized for civilian electricity production, but it has "dual-use" potential and can be modified to produce material for nuclear weapons.

An adjacent reactor at Yongbyon also appears to show signs of operation, according to 38 North, a project of the US-Korea Institute at Johns Hopkins. Satellite imagery from February shows "steam vapor plumes emanating from the generator hall and river ice melt" near the 5 megawatt reactor. The ice melt would likely indicate that the cooling water pipeline has been extended into the river "to conceal the reactor's operational status," 38 North said.

The reactor, which is just upriver from the ELWR, uses pumped-in water from the Kuryong River as its cold water intake and discharges heated water downriver.

"If the reactor is operating again, as the evidence suggests, it means North Korea has resumed production of plutonium presumably for its nuclear weapons program," 38 North concluded.

Analysts say it has long been North Korea's goal to construct a light-water reactor. After failing to source one internationally, it began an indigenous program nine years ago.

In the absence of international inspections (inspectors of the International Atomic Energy Agency were last at Yongbyon in April 2009), it's very difficult to establish the role of such plants, or estimate how much fissile material and nuclear warheads North Korea has accumulated. Estimates published last year suggested North Korea had anywhere from 20 to 60 nuclear weapons.

So extensive and ambitious has the North Korean nuclear program been -- both in terms of weapons and missiles -- that the upcoming summits will, even if successful, be the beginning of a very long process.