The US is monitoring intelligence that North Korea's leader, Kim Jong Un, is in grave danger after apparent heart surgery.
The South Korean Government, however, has denied he is seriously ill.
Kim recently missed the celebration of his grandfather's birthday on April 15, which raised speculation about his well-being.
He had been seen, however, four days before that at a government meeting.
Donald Trump's administration officials are looking into who would be in the line of succession if Kim were to die, according to Bloomberg News.
A reporter for the American network also said the US president's administration received information that Kim had heart surgery last week and is believed to be in poor health.
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 rumours about his health.
Kim was receiving treatment after undergoing a cardiovascular procedure early this month, a South Korean media report said.
North Korea marked the anniversary of the birthday of its founding father and Kim's grandfather, Kim Il Sung, as a
national holiday on April 15, but Kim was not seen in attendance.
Daily NK, a speciality website run mostly by North Korean defectors, cited unidentified sources inside the isolated state saying Kim is recovering at a villa in the Mount Kumgang resort county of Hyangsan on the east coast after getting the procedure on April 12 at a hospital there.
South Korea's Unification Ministry, which handles inter-Korean affairs, declined to comment on the report.
Kim's health has deteriorated in recent months due to heavy smoking, obesity and overwork, the Daily NK report said.
"My understanding is that he had been struggling [with cardiovascular problems] since last August but it worsened after repeated visits to Mount Paektu," a source was quoted as saying, referring to the country's sacred mountain.
Kim left for the hospital after presiding over a meeting of the ruling Workers' Party's politburo on April 11, where Kim was publicly last seen, the report said.
Pyongyang fired multiple short-range missiles last week which Seoul officials said were also part of the Kim Il Sung
birthday celebration.
Such military events would usually be observed by Kim, but there was no KCNA report on the test at all.
