The US has said North Korea could begin recovering material for nuclear weapons "within a matter of weeks"
Satellite images suggest "significant activity" is taking place at a North Korean laboratory that could separate plutonium for nuclear weapons.
The photos appear on 38 North, a US website that monitors sensitive sites in the country.
The website says that during the past five weeks exhaust plumes have been seen two or three times at the radiochemical laboratory complex at Yongbyon.
It suggests buildings there are being heated, but it is unclear why.
Analysis by satellite imagery specialists William Mugford and Joseph Bermudez says: "The plumes suggest that the operators of the reprocessing facility are heating their buildings, perhaps indicating that some significant activity is being undertaken, or will be in the near future.
"Whether that activity will be additional separation of plutonium for nuclear weapons remains unclear."
The lab is where North Korea separates weapons-grade plutonium from nuclear reactor waste.
The country announced its intention to restart nuclear facilities in 2013.
In January, North Korea conducted its latest nuclear test explosion, followed within weeks by a long-range rocket launch.
US intelligence suggests North Korea could begin recovering material for nuclear weapons "within a matter of weeks to months".
Tensions on the peninsula have been fuelled by the start last month of annual South Korea-US military drills.
The North has threatened nuclear strikes on Seoul and Washington.
AFP reports that the latest propaganda video from Pyongyang visualises multiple rocket attacks on Seoul, with a warning that “everything will turn into ashes”.