South Korea believes its northern neighbour has suspended operations at its nuclear power plant in Yongbyon, an official in Seoul has said.
The Yongbyon reactor has been at the centre of the nuclear row
"We are treating this matter very seriously," said Kim Sook, a spokesman for the South Korean Foreign Ministry.
If the plant has indeed been closed, analysts warn that spent nuclear fuel could be removed and reprocessed into weapons-grade plutonium.
North Korea warned in February that it had already developed nuclear weapons.
It also said it would not return to multi-lateral talks aimed at ending its nuclear programmes unless the US changed its "hostile" attitude towards Pyongyang.
Kim Sook, director-general of North American affairs at South Korea's Foreign Ministry, said Seoul was trying to work out the purpose of the recent shutdown.
"We'll have to see what North Korea's intention or its future actions will be," Mr Kim told a local radio station on Monday.
The five-megawatt facility at Yongbyon was ostensibly designed to provide nuclear energy to fuel-hungry North Korea.
It was shut down under a 1994 pact with the US, but Pyongyang reopened it in late 2002 amid escalating bilateral tensions.
In October 2003 the North said it had reprocessed 8,000 nuclear fuel rods at Yongbyon - a claim which, if correct, would have produced enough plutonium to make a handful of nuclear bombs.
North Korea is also suspected of running a separate nuclear development programme based on the enrichment of uranium.
Since 2002, three rounds of discussions involving the US, Russia, the two Koreas, Japan and China have sought to ease tensions on the peninsula, with little success.
A potential fourth round was cancelled earlier this year after the North Korea said it was furious that US Secretary of State Condoleezza Rice had branded the nation an "outpost of tyranny".