The document was signed on 8 July and released yesterday.
The signatories include Switzerland, France, Australia, Britain, Denmark, New Zealand, Norway and the Netherlands.
In the letter, the diplomats expressed concerns about the extrajudicial incarceration in "large-scale places of detention, as well as widespread surveillance and restrictions, particularly targeting Uighurs and other minorities in Xinjiang".
Beijing denies any allegations of torture or political indoctrination, and says the camps are "vocational training centers" created to fight terrorism and combat Islamic extremism.
The decision was down to fears of political and economic retaliation from Beijing, diplomats told Reuters.
China says its treatment of ethnic Muslims in "happy" and "secure" Xinjiang region was a model for other nations to follow, despite a bombardment of Western criticism.
"We urge these countries to respect the facts, discard prejudice, abide by the purposes and principles of the UN Charter and stop politicising human rights issues and intervening in China's internal affairs with the Xinjiang issue", he said.
"We also call on China to refrain from the arbitrary detention and restrictions onfreedom of movement of Uighurs, and other Muslim and minority communities in Xinjiang".
Last month, China's ambassador to the United Nations in Geneva said he hoped Ms Bechelet would take up an invitation to visit.
The detentions are created to eradicate their religious and ethnic identities, Reuters reported Wednesday, citing diplomats and letters. Former detainees have reported cases of torture, forced medication, and rape.
The joint statement comes as the Xinjiang region marks the tenth year since the eruption of ethnic clashes that killed about 200 people.
Top UN officials including Michelle Bachelet have called for the global body to be given access to the camps to investigate allegations of human rights abuses. "The joint statement demonstrates that Beijing is wrong to think it can escape worldwide scrutiny for its abuses in Xinjiang, and the pressure will only increase until these appalling abuses end".
'It is a public politicisation of human rights issues and wantonly interferes in China's internal affairs, ' he added.
China had now effectively contained terrorism and religious extremism in Xinjiang, Vice-Governor Erkin Tuniyaz told the council in an appearance that was sharply criticized by the United States. This is the Chinese regime's stated rationale for rounding up Uyghurs in the camps.
The provision requires that the U.S. Director of National Intelligence Dan Coats submit to congressional intelligence committees a report on activities by Beijing to repress ethnic Muslim minorities in Xinjing, no later than 150 days after the Act's passage.
Somalia: blast, armed attack leaves 10 dead
Security official Abdiweli Mohamed said several heavily armed gunmen forced their way inside the hotel, shooting as they went. Outside of Kismayu, al Shabaab still controls parts of Jubbaland, a region in southern Somalia.