(CNN) - A top official in the Palestinian militant group Hamas lashed out at the son of a Hamas founding member who says in a new book he was an Israeli spy for a decade.

"The Israelis issued this book as a kind of propaganda. When you say that he was working for his enemies, it means that he lies for his own people, and we expect that there are big lies in this book," Osama Hamdan told CNN's Christiane Amanpour in an exclusive interview from Damascus, Syria.

"He's a big liar. He lost his faith. He lost his moralities. And he's working for the enemies who are killing his own people, who are arresting his father, and who are occupying his country," Hamdan declared Wednesday.

In his book, Mosab Hassan Yousef says he gathered information about Hamas terrorist plots and fed them to Israel's domestic security service Shin Bet, and thereby saved countless Israeli and Palestinian lives.

Yousef, in an interview with Amanpour, said he did it because he came to believe that Hamas was practicing "exceptional cruelty" against its members and "killed people for no reason."

Watch the full interview here

He wrote a book, "Son of Hamas," from his new base in the United States, where he has lived since 2007.

CNN could not independently confirm his story, and Israel has refused to comment.

A journalist specializing in intelligence matters on the Israeli newspaper Haaretz, Yossi Melman, said he believes Yousef's story, even though some people say he is exaggerating his role.

"I know that his story is, generally speaking, accurate, (but he) may be slightly over inflating his own role and its importance," Melman told Amanpour.

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