South Korea's government said Tuesday it had no information to corroborate a media report suggesting that Washington is monitoring intelligence indicating North Korea's leader was gravely ill after he reportedly underwent heart surgery this month.

Seoul has not detected any unusual activity or signs coming from Pyongyang about Kim Jong Un's health, according to Kang Min-seok, a spokesman for the Blue House, the office and residence of South Korea President Moon Jae-in.

South Korea's statement comes after a CNN report, citing an unnamed U.S. official "with direct knowledge" of the matter, said the U.S. is keeping an eye on intelligence suggesting that the North Korean leader's health was in "grave danger." A second official cited by CNN said the intelligence about Kim's health was credible. The official acknowledged it was hard to assess how seriously ill he may be.

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The North Korean leader's health has long been a focus of speculation. He's a heavy smoker and is obese. However, North Korea is one of the world's most secretive nations and information about its leader is extremely difficult to verify. Even his age is a matter of dispute, although most analysts think he's probably 36.

CNN's report late Monday followed a story in a Seoul-based website named Daily NK run by North Korean defectors. It reported that the North Korean leader had undergone heart surgery on April 12 and is recuperating at a villa outside the capital, Pyongyang. Daily NK's story is based on a single source inside North Korea.

Media reports about North Korea and its leadership have previously turned out to be inaccurate. Speculation about Kim Jong Un's health has increased in recent days partly because he skipped a recent commemoration connected to the nation's founding.

North Korean state media has not addressed Kim's health. The state-run Korean Central News Agency released a photo of him, dated April 11, which it said shows Kim attending a politburo meeting of the ruling Workers' Party of Korea in Pyongyang. However, neither the content nor the date of the photo could be independently verified.

North Korea is one of only a handful of nations to refuse to report data about coronavirus infections. Negotiations between Washington and Pyongyang aimed at dismantling North Korea's nuclear and ballistic missile programs have stalled amid the Trump administration's refusal to ease crippling sanctions on North Korea.

In March, Kim Jong Un's regime conducted nine missile tests, the most in a single month recorded in a database run by the James Martin Center for Nonproliferation Studies at the Middlebury Institute of International Studies. This month, North Korea launched back-to-back missiles, fired from the ground and fighter jets that splashed down into waters off the coast of Japan. The missiles were fired on the eve of the state anniversary celebrations that Kim Jong Un did not attend in person.