China's representative said worldwide organisations and media who visited Xinjiang had found the situation was different from its portrayal in the West, and that officials from countries behind the letter declined an invitation to visit.
China was urged "to allow meaningful access to Xinjiang" for United Nations and independent global observers.
An estimated one million mostly Muslim ethnic minorities are held in internment camps in the northwest region of Xinjiang - a system that Beijing defends as necessary to counter religious extremism and terrorism.
"We call on China to uphold its national laws and worldwide obligations and to respect human rights and fundamental freedoms, including freedom of religion or belief in Xinjiang and across China", the letter obtained by Reuters stated.
The decision was down to fears of political and economic retaliation from Beijing, diplomats told Reuters.
The Human Rights Watch welcomed the letter and the efforts of the 22 ambassadors of different countries.
The signatories issued the statement as a "letter" at the council and stopped short of seeking a council resolution - a testament to the challenges of building support against increasingly influential China. A United Nations spokeswoman said at the time that the trip, including "full access to Xinjiang", was under discussion. China denies abuse in the detention centers and calls them training schools aimed at combating extremism and providing employable skills.
"We welcome those who truly uphold the principles of objectivity and fairness to come to Xinjiang but firmly oppose any external forces interfering in China's internal affairs, undermining China's sovereignty and territorial integrity through Xinjiang issues", the spokesperson said.
"We have already lodged stern representations with the relevant countries".
It pointedly cites China's obligations as a member of the 47-member state forum to maintain the highest standards.
They urge China to stop detaining minorities and grant them "freedom of movement" within their communities.
While China restricts access and reporting in Xinjiang, Geng said it would welcome a visit by Michelle Bachelet.
FILE PHOTO: An ethnic Uighur woman carries a metal rod as she walks down a main road in the city of Urumqi in China's Xinjiang Autonomous Region, July 10, 2019.
At the start of the three-week session, which ends on Friday, the Xinjiang vice governor responded to the worldwide condemnation of the state-run detention camps by saying that they were vocational centres which had helped "save" people from "extremist" influences.