An Israeli soldier seized by Palestinian fighters more than three years ago described his captivity as an "intolerable and inhumane nightmare" in a 2006 letter to his parents that was made public Wednesday.

In carefully printed script, Sgt. Gilad Shalit reported deteriorating health and deep depression, making an anguished appeal to the Israeli government to release him from his "closed and solitary prison." Shalit, now 23, wrote the 14-line letter three months after gunmen affiliated with the Gaza Strip's Islamist Hamas rulers captured him in a cross-border raid.

The existence of the letter had been known, but his parents had not published its contents. It was leaked to the Israeli news media ahead of the publication of a book that purports through militant sources to chronicle his captivity and Israel's unsuccessful efforts to trade him for Palestinians it holds.

Shalit's captors have not allowed anyone to see him. Three letters and an audiotape relayed to his parents have been the only signs of life from him since he was seized. The most recent was a letter that former U.S. president Jimmy Carter passed to his parents in late 2008.