Analysis of recent commercial satellite imagery indicates that North Korea could be restarting operations at its main nuclear weapons fuel reactor after a five-month hiatus, according to researchers at John Hopkins University.

The photographs, captured between December 24 and January 11, show possible new activity at the Yongbyon nuclear facility in the country's northwest, characterized by possible steam emissions, melted water running off the roofs of the reactor and turbine buildings, and hot water drainage into the mostly-frozen river creating several melt pools.

"The imagery shows that there is renewed activity where there wasn't any in the last five months," Jenny Town, Assistant Director of the US-Korea Institute at Johns Hopkins School of Advanced International Studies, told VICE News. "If the North Koreans have restarted the reactor, it shows they are continuing to build up material for their nuclear weapons program."

The Institute last detected activity at the plant in August, when analysts observed, among other movements, white foam coming out of exhaust pipes. The reason for the temporary shutdown remains unknown, but possible theories include that there was inadequate water supply because of a recent drought, or that the North Koreans were removing some of the old fuel rods that had degraded, Town said.

Analysts believe North Korea has employed Yongbyon — which incorporates the country's only functioning plutonium reactor — to manufacture materials used in previous nuclear tests. While initially a signatory to the 1985 Nuclear Non-Proliferation Treaty, the country unilaterally withdrew in 2003 and conducted its first nuclear test in 2006.

It is not known whether the country currently has the capability to build miniaturized nuclear weapons to tip their intermediate or long-range missiles, which could ultimately take nuclear aggression further across oceans and possibly to US shores.

"We don't have any specific proof that they have miniaturization capabilities," Town said. "But based on [North Korea's] nuclear cooperation with other nations — Syria, Iran, and other countries — we believe they could have the knowledge, but they haven't demonstrated it publicly."

YONGBYON NUCLEAR FACILITY, NORTH KOREA - JANUARY 11, 2015: This is Figure 2. 5 MWe Reactor on January 11, 2015. Note: image rotated. Date: January 11, 2015 -- published on 38 North. (Photo DigitalGlobe/38 North via Getty Images)

Operations at Yongbyon were temporarily put to bed in 2007 as part of an exchange deal for aid announced during six-party denuclearization talks with the US, South Korea, Japan, China, and Russia. But in 2009, the government broke off the talks and banned international inspectors from entering the facility shortly after. It carried out its second nuclear test that same year.

North Korea again agreed with the US to suspend nuclear testing in February 2012 in exchange for food aid, but months later conducted another nuclear test and announced it would restart its 5-megawatt reactor and its uranium enrichment plant in 2013, after relations with America soured following a row over a launched rocket.

When fully functioning, the facility's reactor is thought to be able to produce enough weapons-grade plutonium to make one nuclear bomb a year, Town said.

"We suspect that they have 6-10 bombs worth of plutonium that's been stockpiled," she added. "One more bomb's worth per year may not sound like a lot, but coupled with a presumably growing uranium enrichment program, it means the North is become an ever-increasing nuclear threat."

The North Korean government threatened to launch its fourth nuclear test in November as a knee-jerk reaction to moves by the United Nations to prosecute the country for wide-ranging human rights abuses, as detailed in a report that Pyongyang has repeatedly rejected and even counterpoised with its own glowing report on its rights record.

Possible new activity at the reactor could signal a further crumbling of relations between the reclusive state and the US, which have become increasingly brittle in recent months. In December, the US accused North Korea of conducting a cyber attack on Sony Pictures for producing a film depicting the assassination of leader Kim Jong-un, and placed more sanctions on the impoverished nation. Pyongyang denies it hacked the studio.

The John Hopkins researchers say since the data only spans a two-week period, more analysis needs to be done and that further satellite imagery should give a clearer picture of activity at the reactor site, if any.

Earlier this month, North Korea offered to suspend its nuclear program if the US and South Korea put a cap on their joint military drills, which the North sees as an act of aggression. Washington rejected the proposition, saying the exercises were routine and labeled the offer "an implicit threat."

North Korea claims it is willing to return to six-party negotiations, but on Wednesday, Sung Kim, the US special representative for North Korea policy and former special envoy for the six-party talks, said that the US is "not rushing back into negotiations."

"We want to make sure there is adequate preparation and there is adequate demonstration of commitment by all parties — especially North Korea — to denuclearization, so that if and when we resume negotiations and the six-party talks, we have a much better chance of making some real progress," he added.