A medical aid group says thousands of migrants detained in Greece for lacking the correct paperwork, including some asylum seekers and teenagers, are held in often appalling conditions with lack of access to basic hygiene and regular medical care.

The report released Tuesday by Medecins Sans Frontieres, or Doctors Without Borders, said the detention policy causes "unnecessary suffering and harm."

In 2012, Greece launched a crackdown on illegal migration in which tens of thousands of migrants were rounded up from city streets for identity checks.

The report expressed "grave concerns" and urged Greece to "put an end to the systematic and indiscriminate use of detention" for migrants found without the correct documentation — many often picked up during random checks after living in Greece for years.