Rumors have been swirling that Kim Yo Jong, the sister of North Korean leader of Kim Jong Un and most powerful woman in the country, has fallen from favor.

She was excluded from high-level party talks earlier this month, and wasn't seen at Kim Jong Un's side at his first summit with Russian President Vladimir Putin.

She previously accompanied her brother to important international events, like his two summits with US President Donald Trump.

Kim Yo Jong isn't the only high-level North Korean who seems to have fallen out of favor with her brother: Kim Yong Chol, the country's point person for nuclear negotiations with the US, was excluded from the trip altogether.

The pair's demotions come as Pyongyang takes a harder line against Washington after US-North Korea nuclear talks broke down in February.

Visit Business Insider's homepage for more stories.

Kim Jong Un's sister, who is often referred to as the "princess" of North Korea, was mysteriously absent from her brother's side at his first-ever summit with Russian President Vladimir Putin this week, further fueling rumors of a fall from favor.

Experts speculated earlier this month that Kim Yo Jong, who was considered the most powerful woman in North Korea, had been demoted for an unknown reason.

She was not listed as an alternate member of North Korea's ruling Workers' Party of Korea's (WPK) politburo — the party's top decision-making body — and did not appear at any high-profile events during an important party gathering earlier this month, NK News reported.

Kim Yo Jong arrives at Dong Dang railway station in Vietnam in February 2019 ahead of Kim Jong Un's second summit with US President Donald Trump. Kyodo News via Getty Images

She was also notably absent from footage of her brother's visit to the Russian city of Vladivostok this week for his meeting with President Vladimir Putin, where the pair focused on North Korea's nuclear program.

Read more: Putin broke the habit of a lifetime and didn't show up late for his first-ever meeting with Kim Jong Un

The state-run Korean Central News Agency (KCNA) did not mention whether Kim Yo Jong was going to Russia at all, Agence France-Presse reported.

And while it is possible that she went on the trip — South Korea's Chosun Ilbo and the BBC's Alastair Coleman reported that she left for Vladivostok a day earlier to prepare for the summit — the fact that she wasn't by her brother's side is significant.

Russian President Vladimir Putin, right, toasts with North Korea's leader Kim Jong Un after their talks in Vladivostok, Russia. AP/Alexei Nikolsky, Sputnik, Kremlin Pool Photo

"Princess" Kim's long history of traveling with her brother

Kim rose to fame in 2017 after she was promoted to the head role of the WPK's propaganda department and became an alternate member of the WPK's politburo.

She has since accompanied her brother to multiple high-level international events, including his first summit with South Korean President Moon Jae-in in April 2018, and both his summits with US President Donald Trump in 2018 and 2019.

Kim Jong Un notably used his sister's pen to sign a joint agreement on denuclearization at his first meeting with Trump in Singapore in June 2018.

When she attended the Pyeongchang Winter Olympics in February 2018, she also became the first member of North Korea's ruling Kim family to visit the south since the Korean War in the 1950s.

Read more: Meet North Korea's most powerful woman, Kim Yo Jong: Kim Jong Un's sister who may have just been demoted

Kim Jong Un with Kim Yo Jong during his first meeting with South Korean President Moon Jae-in at the Korean demilitarized zone in April 2018. Korea Summit Press Pool via Reuters

Other high-level demotions suggest a shift away from the US

Kim Yo Jong is not the only high-ranking North Korean who appears to have fallen out of favor with Kim Jong Un.

The North Korean leader's point man for nuclear talks, Kim Yong Chol, was frozen out of the Russia summit altogether, Reuters reported. He is expected to hand over his portfolio in nuclear talks to other diplomats who previously held lower ranks.

Kim is a common Korean surname; the nuclear official does not appear to be related to North Korea's ruling family.

Kim Yong Chol, North Korea's top nuclear negotiator, with US Secretary of State Mike Pompeo at a working dinner in New York City in May 2018. State Department

Kim Yong Chol, a former military intelligence chief with a reputation as a tough negotiator, had been North Korea's point person in nuclear negotiations with the US.

He has met US Secretary of State Mike Pompeo multiple times, and even hand-delivered a letter from Kim Jong Un to Trump at the White House in June 2018.

But despite his long relationship with Washington, he never seemed willing to scale down North Korea's nuclear program in exchange for concessions from the US.

Kim Yong Chol in the Oval Office with Trump in June 2018. Shealah Craighead/White House

Mounting US-North Korea tensions

Kim Yong Chol's apparent demotion suggests that Pyongyang is looking to stop relying on the US and turn to other countries to negotiate its nuclear development.

Pyongyang has taken a hard line against the US recently and has appeared to plow on with its nuclear development after nuclear talks between the two countries in February broke down.

Hours after North Korea claimed to have tested a new tactical guided weapon earlier this month, its foreign ministry issued a scathing statement against Pompeo, saying that he had been "letting loose reckless remarks and sophism of all kinds against us every day."

Read more: North Korea didn't fire a ballistic missile — here's what US intelligence believes it actually tested

During his meeting with Putin, Kim also said the US adopted "unilateral attitude in bad faith at the recent second DPRK-US summit talks," Agence France-Presse cited KCNA as reporting on Thursday.

On Thursday, The Washington Post reported that Pyongyang had demanded $2 million from the US to pay the hospital bill of Otto Warmbier, an American college student who was sentenced to hard labor in North Korea in 2017 after he was accused of stealing a propaganda poster from his hotel during a visit to the country.

Warmbier was returned home in a coma and died shortly after his return to the US. Trump tweeted on Friday that "no money was paid to North Korea for Otto Warmbier, not two Million Dollars, not anything else."

Kim and Trump at their second summit in Hanoi, Vietnam, in February 2019. Those talks broke down and ended early. SAUL LOEB/AFP/Getty Images

North Korea has long argued that the US' policy of "maximum pressure" sanctions harmed mutual relations and has long tried to relax international sanctions against the regime.

Trump said Kim had demanded a full relaxation of international sanctions on his country in exchange for only a few nuclear site closures.

But North Korea's foreign minister, Ri Yong Ho, said Pyongyang had only asked for a partial lifting of sanctions, and that North Korea offered to dismantle some nuclear sites and testing, but the US demanded more.

Read more: Photos show how Trump's 2nd nuclear summit with Kim Jong Un played out in Vietnam

It's not clear if Kim asked Putin to help relax international sanctions on North Korea this week, but it likely would not have worked. Moscow has remained committed to keeping sanctions in place until North Korea dismantles its nuclear program.