Israeli Prime Minister Benjamin Netanyahu is suspected of bribery, fraud and breach of trust, an Israeli police document says.

Police have been questioning Netanyahu for months over the cases but have released few details.

On Thursday, pplice released a gag order on reporting the details of talks that are under way to enlist a state witness. The document says the cases involving Netanyahu deal with "a suspicion of committing crimes of bribery, fraud and breach of trust".

The Israeli daily Haaretz said Netanyahu's former chief of staff Ari Harow has reached an agreement with the prosecution to turn state witness in two corruption cases against Netanyahu that are being heard in court.

The cases relate to gifts allegedly given to Netanyahu and his family by business figures and discussions he had with a newspaper publisher.

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Netanyahu has denied any wrongdoing, and his spokesman said in a statement that the premier was the target of "a witch-hunt, now at its peak, aimed at changing the government.

"This is destined to fail, for a simple reason: Nothing will happen because nothing happened," the spokesman said.

Harow was Netanyahu's chief of staff in 2008 when he was opposition leader. He returned to work with the prime minister in 2014 but resigned a year later amid corruption claims.

The prime minister was questioned earlier this year under caution by police over the allegations.

Although he would not be forced to resign if indicted, it would increase pressure on him to step down after 11 years in office.

Spars on social media

Netanyahu's eldest son Yair, meanwhile, is facing threats of a libel suit.

The 26-year-old lashed out on Facebook at a website run by a liberal think-tank that detailed what it said was his lavish lifestyle at taxpayers' expense.

In the post, Netanyahu alleged that the site was funded by what he claimed were foreign interests, referring indirectly to the New Israel Fund, which he renamed the "Israel Destruction Fund". He signed the post with emojis of a middle finger and a pile of excrement.

Avner Inbar, the chairman of the Molad organisation which runs the website, said they had served Yair with a notice of intent to sue if he does not retract his comments, saying the posts "had no iota of truth to them".

The younger Netanyahu went on to claim that the children of former Israeli leaders Shimon Peres, Ariel Sharon and Ehud Olmert did not come under such scrutiny.

He included an insinuation that one of Olmert's sons had an "interesting relationship with a Palestinian man" that affected national security.

Olmert's son Ariel fired back on Facebook, denying he was gay, dismissing the claims as a fabrication and accusing the younger Netanyahu of "racism and homophobia".

Ariel added that he works for a living, never slept in the prime minister's residence and "on principle, try to pick up my dog's doody" - a reference to a neighbour's account who saw Yair refusing to pick up after his dog at a public park last weekend.

When confronted by the neighbour, Yair gave him the middle finger.

Ariel's older brother Shaul then chimed in, calling Yair a fascist thug.