GENEVA — The Taliban must provide Ryan Corbett, an American held in Afghanistan for nearly two years, with immediate medical care to prevent irreparable harm to his health or even his death, a United Nations expert said Thursday.

“The Taliban must provide Ryan Corbett with medical treatment in a civilian hospital without delay,” said Alice Jill Edwards, the U.N. Special Rapporteur on torture and other cruel, inhuman or degrading treatment or punishment.

Corbett, an aid worker, has been held without charge in conditions “utterly inadequate and substantially below international standards,” she said.

“This is having a significant impact on his physical and mental health, which is declining rapidly,” added Edwards. She said she had raised the issue directly with the Taliban.

“Without adequate medical care he is at risk of irreparable harm or even death,” she said.