(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."
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 overinflating his own role and its importance," Melman told Amanpour.
"He was not the only agent that the Israeli security service had at the time inside Hamas. The intelligence work is a mosaic work. You need many sources."
Melman suggested the book is an embarrassment for the Israeli security services, because they want to protect the names of their sources and agents.
Hamdan though said that the militant group's security people "discovered (Yousef) had some problems, so he was far from any sensitive issues inside Hamas."
He said Israel, through Yousef's book, is trying to create the impression that its security services can go anywhere and touch anyone inside Hamas.
"It doesn't make any sense inside Hamas. There is no disturbance, as maybe some Israelis believe."
Fawaz Gerges, a professor of Middle Eastern politics at the London School of Economics, said Hamas is a very secretive organization that is extremely difficult for a young man like Yousef to penetrate or to infiltrate the military wing.
"We are dealing with an underground world, a universe of espionage, where it's very difficult to disentangle myth from reality," he added. "This is a very, very complex universe where lies are the norm, not the exception."
Gerges said Yousef certainly was an Israeli agent but exaggerates greatly, and his story calls for skepticism. He added that Palestinians with whom he has spoken in recent days say Yousef's claims are "a pack of lies."
Melman, though, insisted the book is not a "pack of lies." He went on to say, "But at the same time, I think he's a bit exaggerating his role and his importance in providing information for Israel and for helping Israel protect its national interest and to protect the safety of our civilians."
Hamdan had a very different picture of what's happening in the Israeli-Arab conflict.
"They (Israel) are the terrorist state in the region, killing the Palestinians under their occupation, like what happened in Gaza last year, what happened in Lebanon in 2006," Hamdan said.
Gerges said the bigger story coming out of the controversy over the book is how Israel has successfully recruited thousands of Palestinians to be Israeli agents.
"This tells you about the fragmentation of Palestinian society. It tells you about the resources and the means by which Israel infiltrates Palestinian society," Gerges said.
"Palestinians have been engaged in a civil war as a result of this espionage because, as we know, many supposed or alleged agents have been killed by both the Palestine Liberation Organization and Hamas."