North Korean leader Kim Jong Un

The US is monitoring intelligence that North Korean leader Kim Jong Un is in grave danger after surgery.

Iran PressAsia: 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 the concerns about Kim's health are credible, but the severity is hard to assess.

Daily NK, an online newspaper based in South Korea that focuses on North Korea, reports that Kim reportedly received a cardiovascular system procedure on April 12.

Kim received the cardiovascular system procedure because of 'excessive smoking, obesity, and overwork', according to the news site, and is now receiving treatment in a villa in Hyangsan County following his procedure.

After assessing that 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 situation, according to the news site. CNN is unable to independently confirm the report.

Gathering intelligence out of North Korea is notoriously difficult -- one of the most challenging targets for US intelligence.

Kim last appeared in North Korean state media on April 11. April 15 -- North Korea's most important holiday, 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.

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