Human rights groups called on Monday for a United Nations investigation into China’s mass detention of Muslims in the western region of Xinjiang, seeking to galvanize an international response to allegations of widespread abuses. The rights organizations, presenting the issue as a test of the United Nations Human Rights Council’s credibility, urged it to set up an international fact-finding mission during its session that starts at the end of February. “The magnitude of abuses allegedly occurring in Xinjiang demand uncompromising scrutiny,” Kenneth Roth, executive director of Human Rights Watch, said in a statement released at a news conference in Geneva.