China has sharply expanded an internment program that initially targeted ethnic Uighur extremists but is now confining vast numbers of the largely Muslim minority group, including the secular, old and infirm, in camps across the country’s northwest.

Up to one million people, or about 7% of the Muslim population in China’s Xinjiang region, have now been incarcerated in an expanding network of “political re-education” camps, according to U.S. officials and United Nations experts.

As...