North Korean leader Kim Jong-il is believed to be recovering from a recent stroke, officials in South Korea say.
Speaking after a late-night briefing, a South Korean spokesman said that the 66-year-old leader was not thought to be in a serious condition.
A North Korean diplomat, however, has dismissed the reports of Mr Kim's ill health as "worthless".
Speculation over Mr Kim's health mounted after he failed to appear at a parade in the capital Pyongyang to commemorate the 60th anniversary of the North Korea's foundation.
Mr Kim has ruled the communist nation since his father, Kim Il-sung, died in 1994. He has not yet named a successor.
Late on Wednesday, intelligence officials told South Korean President Lee Myung-bak that Mr Kim was believed to be recuperating from an apparent stroke.
"At the meeting, Lee was given a report that the North Korean leader has passed a critical moment after suffering a stroke," Yonhap news agency quoted presidential spokesman Lee Dong-kwan as saying.
South Korean lawmakers earlier received a similar briefing at a closed session of parliament.
"Kim suffered either a stroke or a cerebral haemorrhage, but is recovering, the intelligence agency said," opposition lawmaker Won Hye-young told journalists. "Pyongyang is not in a state of administrative vacuum."
"Although Kim is not fit enough for outside activity, he is conscious and able to control affairs," Yonhap news agency quoted him as saying.
Lee Myung-bak has nonetheless told his Cabinet to be prepared for an abrupt change in North Korea's political situation, the agency said.
Mr Kim has not been seen in public since early in August. He has been known to disappear from public view for extended periods before, only to reappear later. This time, however, the reports of ill health have been given added impetus by news that a team of Chinese doctors was recently summoned to examine him.
North Korea has rejected reports that Kim Jong-il is unwell. The North Korean diplomat in charge of relations with Japan, Song Il-ho, dismissed the claims as a "conspiracy plot".
The communist state's deputy leader, Kim Yong-nam, was later quoted, again by Kyodo, as saying there was "no problem".
Concerns about Mr Kim's health come amid an impasse in international efforts to urge North Korea to abandon its nuclear programme.
North Korea agreed in February 2007 to give up its nuclear ambitions in return for aid and diplomatic concessions, but the progress of the deal has been far from smooth.
After a long delay, Pyongyang handed over details of its nuclear facilities in June 2008. In return, it expected the US to remove it from the list of state sponsors of terrorism.
But the two sides cannot agree on a process to verify the information that North Korea handed over and Pyongyang now appears to be starting to reassemble its main nuclear plant.