South Korea is investigating reports of fresh activity at the Yongbyon nuclear facility in North Korea.
N Korea claimed to have restarted work at Yongbyon in February
South Korea's JoongAng Ilbo newspaper quoted US and South Korean officials as saying that a US satellite detected fumes rising from a boiler at the lab.
Unification Minister Jeong Se-hyun said Seoul was trying to confirm this.
Pyongyang said in July that it had reprocessed 8,000 spent fuel rods to extract plutonium, and has since vowed to boost its nuclear deterrent.
JoongAng Ilbo said a US intelligence satellite had detected "signs of vapour and fumes" from a coal-fired boiler linked to a nuclear laboratory at the plant on four days this month.
It said a truck was also spotted in the area where the nuclear reactor is located.
NORTH KOREA NUCLEAR PROGRAMME
Yongbyon: 5-MWt experimental nuclear power reactor and partially completed plutonium extraction facility. Activities at site frozen under 1994 deal. North Korea says restarted in Feb
Taechon: 200-MWt nuclear power reactor - construction halted under 1994 deal
Pyongyang: Laboratory-scale "hot cells" that may have been used to extract small quantities of plutonium
Kumho: Two 1,000-MWt light water reactors being built under 1994 deal - work has been suspended
"We are trying to confirm the activities, but at this stage I have no definitive information to disclose," Mr Jeong told reporters in Seoul.
Officials will want to establish whether the vapour and fumes relate to gasses that are released when nuclear material is reprocessed or extracted from a nuclear facility.
North Korea claims that it restarted Yongbyon, which was mothballed under a 1994 agreement with the US, in February, and has since produced plutonium for nuclear weapons.
The CIA believes North Korea already has one or two in its nuclear arsenal.
But monitoring North Korea's nuclear activities is notoriously difficult, especially since Pyongyang kicked out UN monitors last year.
The latest report of activity at Yongbyon comes as the US and its allies try to get North Korea to resume talks on the nuclear crisis.
The US and its allies last week offered a plan of "co-ordinated steps" to resolving the stand-off, whereby Washington and Pyongyang would stagger their concessions.
But North Korea, fearful of a possible attack by the US, has insisted on simultaneous moves.
Earlier this week, its foreign ministry spokesman said it would not even sit down at the table with Washington and its allies unless it was promised energy aid and the removal of North Korea from the US' list of state sponsors of terrorism - in return for which it would "freeze" its nuclear programme.
US President George W Bush dismissed this proposal, insisting that North Korea end its programme entirely.