The family of an American prisoner of war captured nearly four years ago in Afghanistan says it has received a letter it believes was written by him.

U.S. Army Sgt. Bowe Bergdahl of Hailey, Idaho, disappeared from his base in Afghanistan on June 30, 2009, and is believed held in Pakistan.

His mother and father in Idaho issued a statement on Thursday saying they've received a letter they are confident was written by their son.

In the statement, Bob and Jani Bergdahl say the letter, delivered through the International Committee of the Red Cross, gives them hope that their son is doing as well as can be expected, under the circumstances.

"Our family is greatly relieved and encouraged by this letter," they wrote.

They didn't release excerpts from the letter or detail its content.

They thanked the Red Cross for its help and support — and renewed their plea for his captors to release Bergdahl, who turned 27 on March 28.

"We hope Bowe's captors will again consider his parents' plea to release him, but in the meantime, we ask that you please continue to keep him in good health and allow him to keep corresponding with us," they wrote.

Bergdahl's captivity, which will mark its fourth anniversary later this month, has been marked by only sporadic releases of videos and information about his whereabouts.

In his hometown of Hailey, residents continue to adorn numerous trees and light poles with yellow ribbons, to remember his plight.

Though he's thought by the United States to be held by the Haqqani Network, a Pakistan-based group that President Barack Obama's administration has declared to be a terrorist organization, leaders of the Haqqani said last December that Bergdahl was actually being held by another Taliban group.

Obama called the Bergdahls on Memorial Day weekend in 2012 to reassure them that he and the U.S. military are doing everything in their power to secure his return.

Col. Tim Marsano, an Idaho National Guard spokesman acting as a media liaison for the Bergdahl family, said details from the letter indicate it's genuine.

Though he hasn't seen the letter, it was his understanding that it was a physical letter, not an electronic message, and had been handwritten.

He said the Bergdahl family had been working with the International Committee of the Red Cross for some time, to establish communication with their son. Marsano said this is the first letter exchange, to his knowledge.

"The Bergdahls ... are very pleased they've gotten this result," Marsano said. "They're very thankful to the ICRC."

The International Committee of the Red Cross in Geneva, Switzerland, is acting as a go-between for Bergdahl and his family in Idaho, part of its mission is to protect victims of armed conflict and other violent situations and provide them with assistance.

"We talk to all the parties in a conflict ... to make sure the detainees are kept in good conditions, fed properly, taken care (of) if they have any medical need," said Alexis Heeb, a spokesman in Geneva. "Of course, one of the priorities, whatever a detainees' status, is they should have contact with their families. For us, it's essential that detainees stay in touch with their relatives, wherever they may be in the world."

The Red Cross hasn't been immune to violence.

On Tuesday, its leaders said it was temporarily removing some staff from Afghanistan and curtailing some activities following an attack last week on its offices in the eastern city of Jalalabad.

The attack that killed one guard is considered an aberration, however; the Red Cross has rarely been hit since the war in Afghanistan began in late 2001 and has good relations with all parties to the conflict, including the Taliban, who allow Red Cross representatives to operate in areas under its control.