The Yongbyon nuclear plant in North Korea is likely to have been restarted, according to a US-based research institute.
Citing satellite images, the institute said steam has been rising from the plutonium-producing reactor, suggesting the nuclear facility is either in or nearing operation.
The analysis report on the 38 North website, a specialist institution on Pyongyang affairs, was produced by the US-Korea Institute at Johns Hopkins School of Advanced International Studies.
Kim Jong-un's regime announced its plans to restart the nuclear facility in April 2013, amid tension in the Korean peninsula. North Korea, which is currently reeling under severe sanctions over its contentious nuclear programmes, will be able to make nuclear weapons from the plutonium produced by the facility.
Shortly after North Korea's April announcement, the US said the restarting of the nuclear facility would be "extremely alarming".
North Korea had scaled down its belligerence against South Korea and the US, cooling down tensions in the region. Nonetheless, the latest signs of Kim Jong-un pushing ahead with the defiant nuclear programme have the potential to raise tensions once again.
The five-megawatt reactor was shut down in 2007 under the terms of the nuclear disarmament agreement.
The facility can produce up to 6kg of plutonium a year, according to the report.
"Satellite imagery from 31 August, 2013 shows white steam rising from a building near the reactor hall that houses the gas-graphite reactor's steam turbines and electric generators," it said.
North Korea had already conducted three nuclear tests defying international regulations. The communist nation is believed to possess weapons-grade plutonium, sufficient to make nearly a dozen nuclear bombs.
"We cannot confirm [the report]... but the government is taking a close look at the relevant moves," Cho Tai-young, foreign ministry spokesperson in Seoul, told reporters.