U.S. officials say Kim Jong Un of North Korea may be ill after surgery, but South Korea casts doubt

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...