China's northwestern Xinjiang region is home to more than 10 million members of the Uyghur Muslim minority group.
"We haven't seen things like this since the 1930s, rounding up - some estimations are in the millions - of people and putting them into camps and torturing them", Kozak said. "It's just remarkably terrible", Reuters quotes the ambassador.
Rights experts consider Pyongyang one of the worst offenders in the world, but the Republican president barely speaks about rights violations when he talks about the North these days.
The State Department's annual human rights report, which is presented as a factual, country-by-country description of the situation on the ground, usually does not offer comparisons with past years.
China has denied such allegations, claiming the camps are "reeducation centres".
"But at least we're starting to make them realize there is a lot of global scrutiny on this", he said.
Xinjiang Governor Shohrat Zakir said on Tuesday that China was running boarding schools, not concentration camps, in the country's far western region as the United States ambassador for religious freedom called the situation there "completely unacceptable" and said sanctions against Chinese officials under the Global Magnitsky Act remained a "possibility".
The U.S.is locked in negotiations with China to resolve disputes over trade and intellectual property rights, while President Donald Trump has frequently offered praise of Chinese President Xi Jinping and the relationship they've developed.
It has warned of retaliation if Washington were to target Chen and the USA administration has yet to act despite complaints about its lack of action from US lawmakers.
The State Department report said Chen had replicated in Xinjiang policies similar to those credited with reducing opposition to Communist Party rule in Tibet, where he was previously stationed.
The report says China significantly intensified its campaign of mass detention of members of Muslim minority groups in the Xinjiang Uygur Autonomous Region in 2018.
Saying that the vocational education and training centers in Xinjiang are the right choice, the Chinese diplomat stressed that China will never allow terrorism and extremism to drag Xinjiang back or to take away what the people have earned for a happy life.
"Today, more than 1 million Uyghurs, ethnic Kazakhs, and other Muslims are interned in reeducation camps created to erase their religious and ethnic identities", he said.
And in a semantic change, the report appeared to shift the United States view of Israel's hold on the Golan Heights seized from Syria in 1967, calling it "Israeli-controlled" instead of "Israeli-occupied" as before.
The report also says media reports in Iran suggest at least 20 anti-government protesters were killed in the country, with thousands more arrested. It said it did not know how many children were affected.
The report said government officials had claimed the camps were needed to combat terrorism, separatism, and extremism but worldwide media, human rights organizations, and former detainees reported that security officials in the camps abused, tortured, and killed some detainees.
The report also accused the government of arbitrary or unlawful killings, forced disappearances, torture and arbitrary detention.
Meanwhile, Pompeo's own stance against evil in the world was undercut by the preface to the report, in which he writes: "The policy of this administration is to engage with other governments, regardless of their record, if doing so will further USA interests".