North Korea has released three US citizens detained there, the Financial Times reported Wednesday, citing a South Korean activist.

The three Americans — Kim Dong-chul, Kim Sang-duk, and Kim Hak-song — were released from a labor camp and given health treatment and ideological education in Pyongyang, the report said.

President Donald Trump's hawkish national security adviser, John Bolton, has called for the Americans' release as a way for North Korea to demonstrate its sincerity ahead of a summit with the US.

North Korea has released three US citizens detained there, the Financial Times reported Wednesday, citing a South Korean activist who campaigns for the release of detainees.

The releases would meet some of the US's demands for North Korea to demonstrate sincerity before a historic meeting between President Donald Trump and North Korean leader Kim Jong Un — something that John Bolton, Trump's hawkish national security adviser, reiterated during an interview on Fox News on Sunday.

The three citizens — Kim Dong-chul, Kim Sang-duk, and Kim Hak-song — have been released from a labor camp and given health treatment and ideological education in Pyongyang, the Financial Times report said.

"We heard it through our sources in North Korea late last month," Choi Sung-ryong told the news outlet. "We believe that Mr. Trump can take them back on the day of the US-North Korea summit or he can send an envoy to take them back to the US before the summit."

The Financial Times report said Secretary of State Mike Pompeo spoke with Kim about the detainees during the pair's secretive meeting last month.

In June, Otto Warmbier, an American university student who was detained in North Korea, died days after being returned to his family in the US while in a coma.

The South Korean newspaper Dong-a Ilbo said the three Americans might be being coached to say that human-rights abuses did not occur while they were in North Korean custody.

North Korea operates several prison camps that have been compared to Auschwitz, the former Nazi concentration camp.

If North Korea has indeed released the three Americans and is preparing to give them to the US, it would represent the latest concession Pyongyang has agreed to make ahead of a summit with Trump.

North Korea has signaled a commitment to denuclearization and said it would stop its nuclear and missile tests, though it has dropped a demand for the withdrawal of US forces from the Korean Peninsula and avoided calling for an end to the US's annual military exercises with South Korea as a condition for giving up its nuclear program.