SEOUL, South Korea — Kim Yong-chol, a former North Korean spy master and vice chairman of its ruling Workers’ Party, had been the country’s most internationally visible diplomat in the past year, visiting the White House twice and leading negotiations for North Korean leader Kim Jong-un’s two summit meetings with President Trump.

Now, he has suddenly become the latest example of how a senior North Korean official’s political fortune is made or broken at the whims of Kim Jong-un. This week, leading South Korean newspapers reported Kim Yong-chol’s fall from grace. One of them, the conservative daily Chosun Ilbo, went so far as to report that Mr. Kim had been banished to forced labor, with many of his negotiating team members either executed or sent to prison camps.

South Korean officials and analysts cautioned that it was too early to say with precision what was happening inside Kim Jong-un’s opaque regime. South Korean news media offered differing conjectures, including whether Kim Hyok-chol, the North’s special nuclear envoy to the United States, had been executed by firing squad in March, as the Chosun Ilbo reported, or was still under interrogation.

But they all agree on one thing: Kim Yong-chol and his negotiating team, which had driven Kim Jong-un’s diplomatic outreach toward Washington, have been sidelined, as the North Korean leader sought a scapegoat to blame for his disastrous second summit meeting with Mr. Trump, held in Hanoi, Vietnam, in February.