Police officers in parts of the vast western Chinese region of Xinjiang have been ordering residents to hand over their passports since October, according to residents interviewed by telephone this week and online photographs of orders from local police departments.

At least four official police notices from different areas of Xinjiang, all dated October, have been posted online. They either tell residents to turn in their passports or say that no new passports are available. On microblog platforms, residents from different towns and counties have written that they recently received calls from police officers telling them to bring in their passports.

There is no obvious pattern to the places that are imposing these restrictions; the rules do not appear to be in effect for the entire region. Xinjiang is a frontier territory of mountains, grasslands and deserts that makes up one-sixth of China’s land mass. It has many ethnic groups, but the areas that draw the most attention from the authorities are home to ethnic Uighurs, a group of mostly Sunni Muslims.