The United States on Monday rejected North Korea's threat to keep detaining two American citizens unless a former detainee stops criticizing Pyongyang, saying the freedom of speech is guaranteed in the U.S. and urging Pyongyang to immediately free detainees.

The North's official Korean Central News Agency issued the threat earlier in the day, claiming that Kenneth Bae, a Korean-American missionary who spent two years at a North Korean labor camp, "keeps spouting invectives against the DPRK."

After his release in late 2014, Bae has recently published a book entitled "Not Forgotten," which recounts his experience of imprisonment in North Korea. In a recent speech at Congress, Bae said the communist nation operates like a "big giant cell."

KCNA said the North "will neither make any compromise nor conduct negotiations with the U.S. over the issue of American criminals nor take any humanitarian measure" unless Bae stops criticizing Pyongyang. It also said that "American criminals now in custody in the DPRK will never be able to go back to the U.S."

In response, the State Department said Bae has the right to freedom of speech.

"The welfare and safety of U.S. citizens abroad is one of the highest priorities of the State Department. In the United States, the right to free speech is guaranteed under the constitution," said Ory Abramowicz, a State Department spokesman.

The North is currently holding Otto Warmbier, a U.S. college student, and Kim Dong-chul, a 62-year-old Korean American.

"We urge the DPRK to pardon Mr. Warmbier and grant him special amnesty and immediate release on humanitarian grounds," Abramowicz said. Warmbier was sentenced to 15 years at hard labor in March for trying to steal a propaganda banner from a Pyongyang hotel.

"A representative from the Swedish Embassy last visited Mr. Warmbier on March 2, 2016. We are in regular, close coordination with the Swedes, and the United States government continues to actively work to secure his earliest possible release," the spokesman said.

Kim was sentenced to 10 years at hard labor on charges of espionage and subversion after being arrested in October last year.

But the department official declined comment on Kim's case, citing "privacy considerations."

The Swedish Embassy handles consular matters for the United States in North Korea because Washington and Pyongyang have no diplomatic ties. (Yonhap)