It writes CNN after information from a u.s. official with direct knowledge of the matter.
Kim Jong Un was seen four days earlier, but his recent absence has raised questions about his health.
Kim was receiving remedy after present process the process, a South Korean media report mentioned late on Monday, amid hypothesis over Kim's well being following his absence from a key anniversary occasion this month.
Kim Jong-un last appeared in state media on 12 April "inspecting a pursuit assault plane group" in a handout that is undated.
South Korea's Yonhap news agency said, in response to the reports, that "it's not true" and that there was "no specific trend". After the embarrassing collapse of the Hanoi summit, Kim Yo Jong was reportedly sidelined and not seen in public for several weeks.
"We are monitoring these reports very closely and as you know North Korea is a very closed society, there is not a free press there, they are parsimonious with the information they provide on many things, including the health of Kim Jong Un", O'Brien said.
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"Kim had suffered from inflammation of blood vessels involving the heart from last August, but his condition worsened after he made multiple trips up and down Mt. Paektu recently", a North Korean source is alleged to have told the paper.
Information in north Korea is hard because of the strict control of the regime. The won traded down more than 1% against the dollar even as South Korean government sources said Kim was not gravely ill.
Russia's foreign ministry said in February it donated 1,500 coronavirus test kits to North Korea, and observers say similar kits have also been shipped there from China.
North Korean leader Kim Jong Un is not believed to be critically ill, an official with the Chinese Communist Party's International Liaison Department told Reuters on Tuesday, following media reports about the state of Kim's health.
Alternatively, Kim Yo Jong could remain a figurehead alongside an elite group of senior party figures collectively steering the country through the turmoil of a leaderless country facing both the coronavirus and rising tensions with the United States over its nuclear weapons programme.
If Kim was in a unsafe state, the impact of his absence would be great on the Communist state, which revolves around a personality cult of its leader.
Kim Yo Jong-who now serves as first vice-department director of the Korean Workers' Party Central Committee-first came to worldwide prominence when she represented North Korea at the 2018 Winter Olympics in South Korea.