(CNN)North Korea appears to have resumed construction at a long-dormant nuclear reactor in recent weeks that, if completed, would dramatically increase its capacity to produce plutonium for nuclear weapons, according to new satellite images obtained by CNN and a source familiar with recent US intelligence reporting on the matter.
The satellite images, which were captured by Maxar during April and May of this year, show North Korea has restarted construction of the second reactor at its Yongbyon nuclear complex after years of inactivity, experts at the Middlebury Institute of International Studies who analyzed the photos said.
The reactor is about 10 times larger than the existing nuclear reactor at Yongbyon, which has been operating since the late 1980s.
US officials are also aware of and closely monitoring recent activity at Yongbyon, according to a source familiar with the situation, who noted North Korea is not trying to hide its efforts to restart construction on the reactor in question.
This appears to be an outward demonstration of North Korea's nuclear progress and ambitions, the source said, adding that new construction on the Yongbyon reactor aligns with Pyongyang's aim of proving it's a nuclear armed state.
Experts say it is difficult to estimate how quickly North Korea could complete construction of the reactor.
But once operational, it could allow North Korea to increase its production of plutonium for nuclear weapons by a factor of 10, according to Jeffrey Lewis, a weapons expert and professor at the Middlebury Institute.
Lt. Col. Martin Meiners, a Pentagon spokesman, declined to comment when asked if there is specific intelligence suggesting North Korea has taken new steps to complete construction of the reactor at Yongbyon.
"However, we've been very clear on the threat posed by the Democratic People's Republic of Korea (DPRK) nuclear and missile programs, our commitment to the defense of the ROK, Japan, and the U.S. homeland, and our shared objective of the complete denuclearization of the Korean Peninsula," Meiners added.
North Korea halted construction of the nuclear reactor at Yongbyon in 1994 under the framework of its agreement with the US. At that point the reactor was still years away from completion, Lewis said.
Only very limited construction activities were observed at the site in subsequent years but Lewis told CNN that he and his fellow researches believe the recent satellite images provide "the first unambiguous indicator that North Korea is moving to complete the reactor."
Specifically, the images taken by Maxar show that North Korea is "connecting the secondary cooling loop of the 50 MW(e) reactor to a pumphouse on the river," he said.
"In the image dated April 20, construction equipment is visible, as are what appear to be pipe segments. By May 7, North Korea had buried the pipe," Lewis added.
"The connection of the cooling loop helps explain other activities seen at the 50MW(e) reactor in recent years," Lewis told CNN, pointing to the observed demolition of a building last year that was believed to house a cooling pond for spent fuel.
"Connecting the secondary cooling loop suggests, in hindsight, that the demolition of the apparent spent-fuel building was an early sign that North Korea intends to complete construction of the reactor," told CNN.
The source familiar with the matter also said that there is a lot of preparatory activity required before any country can start construction on a nuclear reactor. "Preparatory activities speak to intent, planning and long held goals," the source added.
CNN reported earlier this month that US military and intelligence agencies assess North Korea could be ready to resume underground nuclear testing in the near term.
The assessment concludes that Kim Jong Un's government is making preparations at another facility, the Punggye-ri nuclear test site, and could be ready to conduct a test by the end of the May.
Signs of personnel and vehicle activity at the site have been seen through satellite imagery, but the officials do not know if the regime has placed nuclear material in one of the underground tunnels at the test site, which the US has been closely watching.
If North Korea conducts a test, it would be the country's seventh underground nuclear test and the first in nearly five years.
CNN's Oren Liebermann and Barbara Starr contributed reporting