Prime Minister Benjamin Netanyahu “loathes” Barack Obama, and his hostile attitude to the US president constitutes a danger to Israel’s well-being, the head of the Israeli opposition charged on Friday night, in a highly unusual acknowledgement of the long-rumored strained personal ties between the two leaders.

In a bitter verbal assault on the prime minister, Labor party chairman Isaac Herzog slammed Netanyahu for failing to listen to the international community, failing to present peace proposals of his own for an accord with the Palestinians, and failing to work properly with Obama.

It was “a tragedy” that Netanyahu had not presented a peace plan, and was instead “dragged” into responding to other proposals, said Herzog. “The second tragedy, that endangers the security of Israel, is his loathing and hostility for Barack Obama,” Herzog went on, describing this as “one of Netanyahu’s gravest failures.

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Herzog, who was minister of welfare under Netanyahu from 2009-2011, was speaking in an interview on Channel 2 news in the aftermath of this week’s formation of a new Hamas-backed Palestinian unity government. Netanyahu had called on the international community to stand up against what he described as a government backed by a terrorist organization, but instead the US led the world in making clear that it would work with the new Palestinian government, and the EU, the UN and much of the rest of the international community quickly followed suit.

Netanyahu and Obama have long been perceived as having a strained relationship, with policy differences emerging over how to stop Iran’s nuclear program, and the prime minister’s expansion of settlements, among other issues. Obama gave an interview which indicated criticism of some of Netanyahu’s key policies just as the prime minister was flying to meet him at the White House in March, and Netanyahu was seen by some in the US as having sought to bolster Mitt Romney’s prospects in the 2012 presidential elections.

But formally Israeli and American leaders have generally insisted that the two work together professionally. Obama took pains to speak of “my friend Bibi,” using the prime minister’s nickname, when he visited Israel last year, and Netanyahu reciprocated by calling him “my friend Barack.” For a figure as prominent as Herzog to use Israel’s most-watched news program to declare that the prime minister loathes the US president was unprecedented.

Sources close to Netanyahu have claimed that Secretary of State John Kerry had promised the prime minister that the US would not work with the new Palestinian government, and had thus breached understandings with Israel. Herzog charged that Netanyahu “does not listen” to the international community, and they don’t listen to him. Under Netanyahu, Israel was now “completely isolated,” he said.

As opposition leader, Herzog receives regular briefings on diplomatic and security issues from Netanyahu and other leading figures. He has been urging relatively dovish members of the governing coalition — notably the Hatnua party led by Tzipi Livni and the centrist Yesh Atid of Yair Lapid — to leave the government and back him. Herzog said Israel needed to negotiate with the Palestinians on the principle of a two-state solution based on the pre-1967 lines, with land swaps and “arrangements” to resolve the contested fate of Jerusalem.

When it was suggested to him that Netanyahu was prepared to go along with such ideas, Herzog retorted, “His mistake is that he’s not put a proposal on the table.

In comments earlier in the week, Herzog had blamed the US and EU recognition of the Hamas-backed Palestinian unity government on the “complete collapse of Israeli foreign policy” under Netanyahu and Foreign Minister Avigdor Liberman. “Netanyahu and Liberman failed to understand the international arena,” he said.

“Netanyahu speaks [but] the world doesn’t listen,” said Herzog Wednesday, adding that the prime minister’s failure to lead a diplomatic process “let Hamas into the West Bank through the front door.”

Herzog warned that if Netanyahu did not act on the diplomatic front, “Israel will lose the support of the international community and the ability to preserve [Israel] as a Jewish and democratic state.”

The opposition leader called on the prime minister to come up with a clear plan to avoid Israel becoming a binational state with a Jewish minority.

“The man who describes himself as strong against Hamas is revealed as being strong at nothing but talking, Herzog wrote in a Facebook post.

Israel has castigated the US over its position, arguing that by maintaining ties with a government supported by a terror group, the US was indicating to PA President Mahmoud Abbas that it was okay to “form a government with a terrorist group.”

“I’m deeply troubled by the announcement that the United States will work with the Palestinian government backed by Hamas,” Netanyahu said Wednesday, noting that the Islamist group has murdered “countless innocent civilians.”

“All those who genuinely seek peace must reject President Abbas’s embrace of Hamas, and most especially, I think the United States must make it absolutely clear to the Palestinian president that his pact with Hamas, a terrorist organization that seeks Israel’s liquidation, is simply unacceptable,” he said.

Earlier Wednesday, Kerry defended a US decision to work with the new Palestinian unity government, despite Israeli criticism, emphasizing that the new Palestinian leadership did not include any Hamas ministers.

Speaking to reporters in Beirut, Kerry said Abbas “made clear that this new technocratic government is committed to the principles of non violence, negotiations, recognizing the state of Israel, acceptance of the previous agreements and the Quartet principles.”

“Based on what we know now about the composition of this technocratic government, which has no minister affiliated to Hamas and is committed to the principles that I describe, we will work with it as we need to, as appropriate.”

While on an unscheduled visit to Beirut, Kerry said: “I want to make it very clear we are going to be watching it (the government) very closely, as we have said from day one, to absolutely ensure that it upholds each of those things it has talked about, that it doesn’t cross the line.”

The new Palestinian cabinet was sworn in Monday, after a surprise reconciliation deal reached in April between Hamas and the PLO.