South Korea says it can't corroborate reports that North Korean leader Kim Jong Un is gravely ill, with an official in the presidential office saying Seoul believes he is with officials traveling outside the capital, Pyongyang.
The official said that "no unusual signs supporting reports about his health conditions have been detected." They also cast doubt on a report by a South Korean-based newspaper that Kim was recovering from surgery in the Mount Myohyang area, saying the leader was not believed to be in that part of the country.
Earlier, a US official with direct knowledge told CNN that Washington was monitoring intelligence which suggested Kim was in grave danger after undergoing a previous surgery. A second source familiar with the intelligence told CNN that the US has been closely monitoring reports on Kim's health.
Kim recently missed the celebration of his grandfather's birthday on April 15, which raised speculation about his well-being. He had been seen four days before that at a government meeting.
Another US official told CNN Monday that concerns about Kim's health were credible -- but the severity is hard to assess.
The most extensive reports on Kim's alleged surgery and poor health came from Daily NK, an online publication based in South Korea staffed in part by defectors from the north. It has a mixed reputation among North Korea watchers, and has published stories about the country which have later been debunked.
According to Daily NK, Kim reportedly received a cardiovascular system procedure on April 12 because of "excessive smoking, obesity, and overwork," and is now receiving treatment in a villa in Hyangsan County following his procedure.
Daily NK reported that after assessing Kim's condition had improved, most of the medical team treating him returned to Pyongyang on April 19 and only part of them remained to oversee his recovery. CNN is unable to independently confirm the report.
A South Korean source told CNN Monday that the country's top leaders are aware of reports about Kim's health status but cannot independently verify details published by Daily NK. The source acknowledged that Kim's issues related to weight and smoking are well known but made clear they are waiting for more information.
South Korea's presidential Blue House said in a media statement that it had nothing to confirm on reports about Kim's health and "no unusual signs" had been detected inside North Korea.
South Korea's Unification Ministry and Defense Ministry declined to comment on speculations about Kim's health.
The National Security Council and Office of the Director of National Intelligence declined to comment when contacted by CNN on Monday. CNN has also reached out to the CIA and the State Department for comment.
Speculation over Kim's health has raised concerns over the succession plan in North Korea and possible instability should the country's young leader die.
US officials have been reaching out to North Korean experts -- particularly those who have studied the Kim regime -- to talk about contingency planning, a source familiar with the communications told CNN.
Still, the situation remains murky as gathering intelligence out of North Korea is notoriously difficult -- one of the most challenging targets for US intelligence.
North Korea tightly controls any information surrounding its leader, who is treated almost like a deity within the country. His absences from official state media often spark speculation and rumors about his health.
North Korea has no free press and is often a black hole when it comes to the country's leadership. Analysts are heavily reliant on scanning state media dispatches and watching propaganda videos for clues.
Kim last appeared in North Korean state media on April 11. The country's most important holiday, the Day of the Sun, on April 15 -- the anniversary of the birth of the country's founding father, Kim Il Sung -- came and went without any official mention of Kim Jong Un's movements.
Thae Yong-ho, a former North Korean diplomat who defected and made history last week after being elected to South Korea's parliament, said it was unprecedented for Kim not to attend.
"We will have to calmly monitor this issue and the South Korean government needs to monitor situations closely and better be in full preparation for any possible emergency situations in the North," he added.
Experts are unsure of what to make of Kim's absence from any festivities celebrating his grandfather. When North Korean leaders have not shown up to these important celebrations in the past, it has portended major developments. But it has also turned out to be nothing.
"There have been a number of recent rumors about Kim's health. If Kim is hospitalized, it would explain why he wasn't present on the important April 15th celebrations," said Bruce Klingner, a senior research fellow at the Heritage Foundation and former CIA deputy division chief for North Korea. "But, over the years, there have been a number of false health rumors about Kim Jong-un or his father. We'll have to wait and see."
Kim Jong Il's absence from a parade celebrating North Korea's 60th anniversary in 2008 was followed by rumblings that he was in poor health. It was later revealed he had a stroke, after which his health continued to decline until his death in 2011.
Thae said news of Kim Jong Il's death took more than 51 hours to become public. "On that day, at the foreign affairs department in North Korea, it was business as usual until we were told at 11 a.m. to gather at a hall by noon," he said.
The late leader's son also disappeared from the public eye for more than a month in 2014, which prompted speculation about the younger Kim's health. Kim Jong Un returned sporting a cane, and days later South Korean intelligence said that he had a cyst removed from his ankle.
"It's easy to be wrong on this one," said John Delury, a professor of international relations at Yonsei University in Seoul.