America's leading human rights organisation has accused Israel and its supporters of an "organised campaign" of false allegations and misinformation, including "extremely personal attacks" on its staff, in an attempt to discredit the group over its reports of war crimes in Gaza.

Human Rights Watch (HRW) ties the campaign – which has included accusations that the group's reports on the Jewish state are written by "anti-Israel ideologues" and that it has sought funds from Saudi Arabia – to a statement by a senior official in the Israeli prime minister's office in June pledging to "dedicate time and manpower to combating" human rights organisations.

The criticism began with Israeli pressure groups and rightwing blogs, but in recent weeks it has drawn the support of influential individuals such as Elie Wiesel, the Holocaust survivor and Nobel peace prize winner, and HRW's own founder, Robert Bernstein, who said the organisation's reports were "helping those who wish to turn Israel into a pariah state". He called on HRW to focus more on abuses by Arab governments.

Iain Levine, HRW's programme director, said that while the organisation had long attracted criticism, in recent months there had been significant attempts to intimidate and discredit it.

"I really hesitate to use words like conspiracy, but there is a feeling that there is an organised campaign, and we're seeing from different places what would appear to be co-ordinated attacks ... from some of the language and arguments used it would seem as if there has been discussion," he said."We are having to spend a lot of time repudiating the lies, the falsehoods, the misinformation."

Spearheading some of the criticism is NGO Monitor in Jerusalem, an Israeli group funded by wealthy US donors which includes Wiesel on its advisory board. It has accused HRW staff of having a "political agenda" to attack Israel.

Criticism has particularly focused on the director of HRW's Middle East division, Sarah Leah Whitson, over a visit to Saudi Arabia.

NGO Monitor accused Whitson of attempting to raise money from Saudi officials by highlighting HRW's criticism of Israel, a charge also made in a comment piece for the Wall Street Journal online that was subsequently widely distributed by the most powerful of the pro-Israel lobby groups, the American Israel Public Affairs Committee (Aipac). Shortly afterwards, the director of policy planning in the Israeli prime minister's office, Ron Dermer, denounced Human Rights Watch.

"We are going to dedicate time and manpower to combating these groups; we are not going to be sitting ducks in a pond for the human rights groups to shoot at us with impunity," he said.

Levine said that Whitson's visit to Saudi Arabia was similar to trips by other HRW officials to Tokyo, Johannesburg and Tel Aviv to win the support of individuals interested in supporting human rights in their own countries and abroad.

"This idea that somehow the Saudi government is going to be able to influence us is nonsense. It's a cardinal principle of the organisation that we don't take government money," he said.

But Levine added that Dermer's threat marked the escalation of the campaign against HRW.

"It was clear that you had a new government in Israel under Binyamin Netanyahu with a harder right approach. He certainly recognised that the criticisms of Israeli conduct in Gaza from a humanitarian law perspective was extremely politically damaging," he said.

Levine said he believes many of the attacks were aimed at distracting attention from the report of the UN investigator, Richard Goldstone, which was highly critical of Israel's killing of civilians in its three-week attack on Gaza that started last December. Goldstone is a former member of the HRW board and the group has strongly backed his report.

"We have been under enormous pressure and tremendous attacks, some of them very personal, as have been the attacks against Richard Goldstone with really vituperative language used to describe him: obsequious Jew, self-loathing Jew and all the rest of it," said Levine.

HRW came under renewed criticism last month from its founder, Robert Bernstein, in an opinion article in the New York Times in which he accused it of criticising Israel more than undemocratic governments in the rest of the Middle East.

"Human Rights Watch has lost critical perspective on a conflict in which Israel has been repeatedly attacked by Hamas and Hezbollah, organisations that go after Israeli citizens and use their own people as human shields," he wrote.

Bernstein accused HRW of basing its accusations against Israel on the testimony of Palestinian "witnesses whose stories cannot be verified and who may testify for political advantage or because they fear retaliation from their own rulers".

Levine said that Bernstein went public only after the HRW board rejected his call for a change in direction.

A few days later, Wiesel and others published a letter in the Guardian drawing attention to Bernstein's article, accusing HRW of playing a "destructive role" and calling for a review by the organisation's board.

In September, HRW was shaken by accusations that its military expert and collector of war memorabilia, Marc Garlasco, is a Nazi sympathiser after describing an SS jacket as "so cool" in comments on a blog. Both he and HRW vigorously deny the charge, but Garlasco has been suspended pending an investigation.

At the time, Levine called the attacks on Garlasco the latest salvo in the Israeli government's campaign "to eliminate the space for legitimate criticism" of the Israeli military.