South Korea is investigating but has yet to confirm reports of fresh activity this month at North Korea's main nuclear centre at Yongbyon.
A South Korean newspaper quoted US and South Korean officials as saying a US intelligence satellite detected fumes rising from a coal-fired boiler at the nuclear lab at Yongbyon. The fumes were traced on four days this month.
Yongbyon, about 60 miles north of the capital Pyongyang, contains a nuclear reactor and a plutonium reprocessing plant at the centre of the year-long crisis over the secretive communist state's attempts to build nuclear weapons.
"We are trying to confirm the activities, but at this stage I have no definitive information to disclose," Unification Minister Jeong Se-hyun told reporters at his weekly news conference in Seoul today.
The paper quoted Seoul officials as saying the fumes were detected on December 2nd, 3rd, 4th and 7th, and that a truck was spotted travelling in and out of the premises of Yongbyon's five-megawatt nuclear reactor on December 3.
The latest report comes as the United States, South Korea, Japan, China and Russia are trying to convene a second round of six-way talks on the nuclear dispute with North Korea to follow an inconclusive first round held in Beijing in August.