UN ambassadors from 37 countries released a letter on Friday defending China's treatment of Uighur and other minorities in the Xinjiang region, in direct response to Western criticism earlier this week.

Envoys from across the EU — along with Australia, Canada and Japan and New Zealand — had earlier co-signed a text denouncing China's conduct in Xinjiang, where one million people, mostly ethnic Uighurs, are reportedly being held in internment camps.

On Friday a diverse group of states — including Russia, Saudi Arabia, Nigeria, Algeria and North Korea -- replied on Beijing's behalf.

"We commend China's remarkable achievements in the field of human rights," said the letter, also signed by Myanmar, the Philippines, Zimbabwe and others.

"We take note that terrorism, separatism and religious extremism has caused enormous damage to people of all ethnic groups in Xinjiang," it said.

Rights groups and former inmates describe the internment sites in Xinjiang as "concentration camps" where mainly Muslim Uighurs and other minorities are being forcefully assimilated into China's majority ethnic Han society.

Echoing China's defense of the camps, Friday's letter described them as "vocational education and training centers."

"Now safety and security has returned to Xinjiang," it said.

The group of ambassadors asked for the letter to be recorded as an official document of the Human Rights Council, which wrapped up its 41st session in Geneva on Friday.

The Western diplomats had made the same request.

Beijing on Thursday dismissed the Western letter as "slander."

The tit-for-tat open letters is rare at the UN's top rights body, where states typically try to hammer out formal resolutions during closed-door negotiations.

After initially denying their existence, Beijing has gone on a public relations blitz in a bid to counter the global outcry against what it calls "vocational education centers" in Xinjiang.

Since last October, the local government has also organised tours of the camps for diplomats and media outlets.

UN human rights chief Michelle Bachelet has requested a fact-finding mission to Xinjiang.

Beijing has said she is welcome, but the rights office has stressed that a visit will be only possible on certain conditions -- including unfettered access to key sites.