The nuclear facility used by North Korea to produce atomic bombs is still in operation, a senior intelligence official was reported as saying on Monday.
North Korea agreed to close a five-megawatt nuclear reactor in Yongbyon and invite back International Atomic Energy Agency (IAEA) inspectors in exchange for diplomatic and economic incentives under a six-nation agreement last month.
"The five-megawatt reactor in Yongbyon is in normal operation at present, considering a constant stream of steam spotted from its cooling tower," an unidentified senior South Korean intelligence official told Yonhap news agency.
China's state-run television news reported that North Korean officials at six-nation talks in Beijing had indicated Pyongyang had already begun preparatory work to shut down the Yongbyon reactor.
The South Korean official, however, said steam coming from the cooling tower showed the reactor was still working.
"Therefore, North Korea's suspension of the Yongbyon reactor will be confirmed by the halt of its cooling-tower steam or an on-the-spot inspection by IAEA inspectors," he was quoted as saying.