The United States has identified the soldier shown on a Taliban video as Private Bowe Bergdahl, and has condemned the images as violating international law.
The video, released on Sunday, shows Private Bergdahl in traditional Afghan dress, being prompted in English by his captors to call for US forces to be withdrawn from Afghanistan.
The US military confirmed the identity of the soldier, saying he served with the 1st Battalion of the 501st Parachute Infantry Regiment. His identifying dog tags are displayed to the camera by his captors in the video.
"We condemn the use of this video and the public humiliation of prisoners. It is against international law," US military spokesman Colonel Greg Julian said. "We are doing everything we can to return this soldier to safety."
The US military has been distributing leaflets this week seeking the release of Private Bergdahl, missing since late June.
Military spokeswoman Captain Elizabeth Matthias said it was the first case she was aware of in which a US service member was held captive by the enemy in Afghanistan, although there have been similar cases in Iraq.
In the video, Private Bergdahl appeared with his head shaven and a slight beard, wearing traditional grey, loose-fitting Afghan shalwar kameez clothing.
He appeared to be in good health and is shown drinking tea and eating bread and rice.
"I am scared. I'm scared I won't be able to go home. It is very unnerving to be a prisoner," he says. "I have my girlfriend who is hoping to marry. I have my grandma and grandpas. I have a very, very good family that I love back home in America."
A voice off camera prompts: "Miss them."
The soldier continues: "And I miss them every day that I'm gone. I miss them and I'm afraid that I might never see them again and that I'll never be able to tell them that I love them again. I'll never be able to hug them."
Later, the voice prompts: "Any message to your people?"
"Yes. To my fellow Americans who have loved ones over here, who know what it's like to miss them: you have the power to make our government bring them home," Pvte Bergdahl says.