Kim Jong Un Reported ill After Surgery

The South Korean government cast doubt Tuesday on rumors that North Korean dictator Kim Jong Un was gravely ill, but U.S. officials said he may be incapacitated after heart surgery.

Speculation about Kim's health has mounted after he failed to attend his late grandfather's birthday celebrations on April 15, the most important date in North Korea's calendar, known as "Day of the Sun."

The rumors accelerated Tuesday when the South Korean website Daily NK cited one unnamed source as saying Kim, who is believed to be 36, had undergone heart surgery and was recuperating in a villa outside the capital Pyongyang.

Several U.S. officials told NBC News that U.S. intelligence reporting indicates that Kim recently had cardiovascular surgery and has been out of public view for days. Some officials said the intelligence suggests Kim may be incapacitated. U.S. intelligence and agencies and the military are working to figure out his health status, officials said.

As a result of the rumors, South Korean financial markets took a dip and its currency weakened against the dollar.

But later on Tuesday the office of South Korean President Moon Jae-in denied that Kim was ill, saying that it could confirm he was at work and "currently touring provincial areas with his close aides."

The office said it had no "evidence to support speculation about his ill health."