The official also said there was no indication the North had collected $25m from Macau bank accounts that were frozen as a result of US action and then released last week.
A February 13 six-party deal requires Pyongyang to shut down and seal the Yongbyon facility in exchange for fuel and security guarantees.
A file satellite image of the Yongbyon nuclear
South Korean media reports on Tuesday had quoted South Korea's intelligence service as saying that US spy satellites had spotted "unusual" activity around North Korea's Yongbyon nuclear reactor, indicating Pyongyang was preparing to shut it down.
The North missed a Saturday deadline to halt work at the reactor and allow UN inspectors to verify and seal the facility under a six-nation agreement signed in Beijing in February.
Pyongyang has not released any statement since the deadline passed.
However, it said last week that honouring its pledge was contingent on the release of $25m frozen in a separate financial dispute after the US blacklisted a Macau-based bank where North Korea had accounts.
The funds were allegedly used in money laundering and counterfeiting.
The US says the money was freed for withdrawal last week, but it is unclear when the North will move to get its money.