The U.S. spied on Israel's military bases and secret communications in 2012 out of fear that Israel may attempt to attack Iranian nuclear sites, according to a Wall Street Journal report based on testimony from nearly two dozen senior U.S. and Israeli officials.

At one point, senior U.S. officials learned that Israel violated Iranian airspace in what was interpreted as a trial run for a commando strike on the Fordow nuclear site, Iran's most heavily fortified facility, buried deep inside a mountain inside an Islamic Revolutionary Guards Corps base.

In response to the dry run flight, the White House sent a second aircraft carrier to the Middle East out of fear that Israel could start a regional war and U.S. fighter jets were readied "in case all hell broke loose," a senior U.S. official told the WSJ.

The report shows that the two allies had grown increasingly suspicious of each other, noted Haaretz. Instead of handling differences diplomatically, both countries kept their intentions secret and used their spy agencies to see what the other was up to. 

The Obama administration had already begun its nuclear negotiations with Iran in secret to prevent Israel from undermining American foreign policy and sabotaging the negotiations.

Both countries had worked together for nearly a decade to keep Iran from developing a nuclear bomb, but they had different opinions on how to best do that, with options including diplomacy, covert action or military strikes.

"Mossad leaders compared the covert campaign to a 10-floor building: The higher the floor, they said, the more invasive the operation," the WSJ reported. "CIA and Mossad worked together on operations on the lower floors. But the Americans made clear they had no interest in moving higher-Israeli proposals to bring down Iran's financial system, for example, or even its regime."

In one covert operation run unilaterally by Mossad, an Iranian nuclear scientist was assassinated, according to U.S. officials.

The Israeli Prime Minister eventually pivoted towards a military strike and went as far as to brief the U.S. on an attack plan: "Cargo planes would land in Iran with Israeli commandos on board who would 'blow the doors, and go in through the porch entrance' of Fordow, a senior U.S. official said. The Israelis planned to sabotage the nuclear facility from inside," the WSJ reported.

Israel also attempted to obtain a 30,000-pound bunker-busting bomb from the U.S. military, which would be dropped on Fordow, according to Gary Samore, who served as Obama's White House coordinator for arms control and weapons of mass destruction.

The U.S. saw the military options as a big mistake and began leaning towards diplomacy, also turning down Israeli requests for more covert measures.

The interim nuclear agreement was announced in November 2013, and the Israelis reportedly "felt blindsided" by the terms, according to Samore. As talks progressed towards a final accord, the Israelis stepped up their spy efforts and voiced their utter disapproval of the terms of the agreement. They were eventually unsuccessfully at preventing U.S. lawmakers from approving the deal.

Now, as the finalized deal is on the verge of full implementation, U.S. and Israeli officials have pledged to work together to monitor Iran's compliance with the international agreement. But it's unclear how the U.S. would respond to Israeli attempts to sabotage Iran's nuclear facilitates, as there is a clause in the deal specifically saying major world powers will help Iran secure its facilities against such attacks.

"If we become aware of any Israeli efforts, do we have a duty to warn Iran?" former director of the CIA Michael Hayden said. "Given the intimacy of the U.S.-Israeli relationship, it's going to be more complicated than ever."