China sent a team including doctors and senior diplomats to advise its neighbor and longtime ally, Reuters reported on Saturday, citing three people familiar with the matter. Meanwhile, a train resembling one long used by North Korean rulers was parked last week near a coastal leadership compound in Wonsan, according to an analysis of satellite imagery released Sunday by the website 38 North. A prominent South Korea adviser also rejected the notion that Kim was ailing or dead.

“Our government position is firm,” Moon Chung-in, a foreign policy adviser to South Korean President Moon Jae-in, told CNN. “He has been staying in the Wonsan area since April 13” with “no suspicious movements” detected.

The latest developments shed little immediate light about the 36-year-old ruler, who U.S. officials said they were told had been in critical condition after cardiovascular surgery. Kim — a heavy smoker who has gained considerable weight since taking power in 2011 — hasn’t appeared in state media for two weeks, and missed April 15 birthday celebrations for his late grandfather, state founder Kim Il Sung, for the first time.

The episode renews longstanding questions about the stability of a regime built on iron-fisted authority and a cult of personality for Kim, who has no known successor. Health scares have been a common occurrence over the years, and the leader’s medical condition is a closely-guarded subject in one of the world’s most secretive states.

Any leadership change in North Korea could increase the threat of instability on China’s border and raise questions about control of the country’s expanding nuclear arsenal. Kim has also been central to U.S. President Donald Trump’s so far unsuccessful efforts to get him to reduce his weapons stockpile.

The mystery sent journalists, diplomats and non-proliferation experts scouring satellite images, state news outlets and unverified social media feeds for clues about Kim. The hashtag #kimjongundead was the among the top subjects trending globally Saturday on Twitter. The name of his younger sister Kim Yo Jong — a potential successor — was also trending in the U.S.

“Information about the serious state of health of our Marshal Kim Jong Un is false and malicious,” Korean Friendship Association head Alejandro Cao de ­Benos said in a tweet Saturday. Cao, who was featured in the 2015 documentary “The Propaganda Game,” didn’t say where he got the information and declined further comment when contacted by Bloomberg News.

Kim similarly disappeared from state media for six weeks in 2014, prompting speculation that he had been sidelined by gout, an ankle injury or was even overthrown in a coup. He subsequently showed up walking with a cane during a visit to a new residential block.

A delegation led by a senior member of the Chinese Communist Party’s International Liaison Department — which manages relations with North Korea — departed Beijing on Thursday, Reuters said, adding it wasn’t able to immediately determine what the trip signaled about Kim’s health. Officials with the U.S. National Security Council declined to comment on Saturday.

Speculation about Kim’s condition accelerated after Seoul-based news site Daily NK, reported April 20 that the North Korean leader was recovering from surgery, citing one unidentified person inside the country. CNN reported a short time later that Kim was in “grave danger.”

Trump has since said he believed the CNN report was “incorrect,” saying he heard “they used old documents.” South Korean officials said repeatedly last week that they believed Kim was conducting “normal activities” in a rural part of the country assisted by close aides, and that they detected no unusual movements by the regime.

The presence of Kim’s train in Wonsan, about 230 kilometers (143 miles) east of Pyongyang, potentially fits with the South Korean account. The area has received intense interest from Kim in recent years, hosting an expanding leadership compound, a large tourism development project and numerous weapons tests.

The train, which Kim took to his failed summit with Trump in Hanoi, arrived at a nearby railway station on or before April 21, 38 North wrote, citing satellite imagery. The train was spotted again at the station on April 23, and appeared to be repositioned for departure.

North Korea’s eastern coast saw a burst of military activity on April 14, including cruise missile tests and fighter jet maneuvers, that at the time appeared to be timed to coincide with South Korean parliamentary elections. The next day, Kim was absent from events marking his grandfather’s birthday, known as the Day of the Sun and North Korea’s most politically significant holiday.

Although state media has continued to publish messages exchanged between Kim and dignitaries, such as a letter sent to Syrian President Bashar al-Assad on Wednesday, reports included no fresh images and made no mention of new events attended by the leader. Kim was similarly missing from events Saturday marking the anniversary of the Korean People’s Revolutionary Army, although he also didn’t attend the events last year.

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