The relationship between Benjamin Netanyahu and Barack Obama, the leader of his nation's most crucial ally, is now frankly, transparently, poisonous.

Worse, their personal animosity has metastasised to infect their alliance.

Such is their mutual mistrust that the former US ambassador to Israel, Martin Indyk, told CNN he feared for how the two might react should a crisis erupt on their watch, adding that he was disturbed to see that many strong supporters of Israel from the Democratic Party had boycotted the speech.

By accepting the Republican invitation to address Congress just a fortnight before he faces his own election, Netanyahu has politicised an alliance that has had enduring bipartisan support even in polarised Washington, says another expert on the alliance, American University's Guy Ziv.

The effects of this are unknowable but real, he told Fairfax Media. Netanyahu may not now have the rapid open-door access to the White House that Israel's leaders have depended on in emergencies, he says.

Nor will the US be as willing to share intelligence on the ongoing negotiations with Iran as it has been, he believes.

Though the tension between Obama and Netanyahu has never been secret, the angry theatre of the two leaders trading insults in the US capital is unprecedented, says Ziv.

When the Israeli prime minister appeared to make his address to a joint sitting of Congress on Tuesday morning, his reception was ecstatic and thunderous, at least from his Republican hosts.

For nearly an hour Netanyahu stood at the lectern from which Obama makes his annual State of the Union addresses and traduced the President in a speech explicitly designed to subvert the White House's key foreign policy goal - negotiating an end to Iran's nuclear weapons program.

Stripped of their very thin diplomatic veil, his words portrayed Obama as naive and weak. He likened Iran to Nazi Germany – and by implication, Obama to British prime minister and "appeaser" Neville Chamberlain.

An hour or so later and a handful of blocks down Pennsylvania Avenue, Obama was meeting with his new Defence Secretary, Ashton Carter, when he took questions from reporters about the address. He was blunt, grim and expansive in response.

He had been too busy dealing with European allies on the Ukrainian issue to watch Netanyahu's address, he said, but he had checked the transcript.

Netanyahu, he said, had failed to offer anything new or propose any alternative means to end Iran's nuclear weapons development.

But Obama went further, saying that in the past Netanyahu had been wrong in suggesting that Iran had failed to abide by the terms of an interim deal. He described a man who was not only intransigent, but willing to exaggerate his case.

Obama's proxies reinforced this message. A source identified by CNN only as an angry senior White House official said Netanyahu had proposed "literally not one new idea; not one single concrete alternative; all rhetoric, no action".

"Where is the alternative? Simply demanding that Iran completely capitulate is not a plan, nor would any country support us in that position. The prime minister offered us no concrete action plan."

The Democratic leader of the House of Representatives, Nancy Pelosi, joined the fray, saying: "The unbreakable bonds between the United States and Israel are rooted in our shared values, our common ideals and mutual interests.

"That is why, as one who values the US-Israel relationship, and loves Israel, I was near tears throughout the Prime Minister's speech - saddened by the insult to the intelligence of the United States as part of the P5+1 nations, and saddened by the condescension toward our knowledge of the threat posed by Iran and our broader commitment to preventing nuclear proliferation."

In the past the two leaders have managed to remain outwardly diplomatic, even while their administrations betrayed the tensions between them.

Last October it was revealed that a White House official had called Netanyahu a "chickenshit" who was more interested in political posturing than de-escalating tensions in the Middle East.

Obama's chief diplomat, John Kerry, was labelled "obsessive" and "messianic" by Netanyahu's defence minister, Moshe Yaalon, for his pursuit of peace between Israel and the Palestinians.

Israel has also developed a habit of marking significant meetings with senior American officials by announcing new settlements on occupied land. After these occasions apologies were made, and the work of the relationship went on.

This time, says Ziv, Netanyahu might have gone too far.