Initial photos from January 18 showed preparations were underway to to restart the reactor at Yongbyon, having previously unloaded spent fuel rods for reprocessing to produce additional plutonium for its nuclear weapons stockpile.
"Imagery from January 22 shows a water plume (most probably warm) originating from the cooling water outlet of the reactor, an indication that the reactor is very likely operating," it said in a report.
The report - put out by the US-Korea Institute at Johns Hopkins University's School of Advanced International Studies (SAIS) - said it's impossible to know at what power level the reactor is running but acknowledged that, "it may be considerable."
North Korea has maintained its nuclear programme and continued to develop its missile programme in violation of repeated rounds of international sanctions.
Indeed, there is growing speculation that the North is planning another missile test – in defiance of UN resolutions.
Shirtless drills in snow
In response, hundreds of marines from the US and South Korea have been conducting shirtless, hand-to-hand combat drills on snow covered ski slopes in Pyeongchang, which is set to host the 2018 winter Olympics.
"US Marine Corps and ROK (Republic of Korea) Marine Corps partnered together at every level to build a camaraderie and friendship of the two countries' militaries but also to increase our proficiency in the event where we have to fight a war together," US Captain Marcus Carlstrom told journalists.
The training began January 15, and will conclude on February 3. Pyeongchang lies about 180 km (115 miles) east of Seoul.
North Korea's state-run media dismissed the exercises, but warned of retaliation.
"The colonial puppet forces, no more than a rabble, are keen on escalating the tension and the moves to ignite a war at a time when even their American master is at a loss how to cope with the DPRK's powerful nuclear deterrent," North Korea's MinjuJoson newspaper, quoted by the KCNA news agency, said.
North Korea's official name is the Democratic People's Republic of Korea, or DPRK.
Acting South Korean President Hwang Kyo-ahn said Monday that the deployment of a US anti-missile defense system should not be delayed in the face of the growing North Korean nuclear missile threat.
Yonhap News Agency reports that South Korean Defense Minister Han Min-koo ordered thorough military readiness, saying on Friday that North Korea's nuclear weapons and missiles were "a direct and substantive threat."