For months, Israeli spy officials were reportedly wringing their hands over an urgent notice from U.S. intelligence officials: Proceed cautiously when sharing top-secret material with Donald Trump.

Their misgivings were about how the 45th U.S. president might mishandle sensitive foreign intel, according to a January report by Israel's respected Yedioth Ahronoth newspaper.

Those very fears bore out in a public fashion this week, as several media outlets reported that Israel — a crucial espionage partner in the Middle East — was the source of highly secret material that Trump declassified for two Russian diplomats.

Trump meets with Lavrov, Kislyak

Trump speaks with Russian Foreign Minister Sergey Lavrov, left, and Russian Ambassador Sergey Kislyak, right, during a meeting in the White House last week. Reports say the U.S. president spilled highly classified data while bragging about the intel he receives. (Russian Foreign Ministry/EPA)

The source of the "code-word" intel, which The Washington Post described as concerning the inner workings of ISIS, could now be in jeopardy as intelligence experts warn of a deep chill over espionage partnerships abroad.

In a tweet Tuesday, the president seemed to confirm reports that he declassified the material while meeting with Russian diplomats last week in the Oval Office.

Trump contended it was within his right as president to show "facts" about terrorism with his Russian guests.

Reports from the New York Times and NBC News that Israel was the partner who provided the classified data on ISIS were disturbing for U.S. intelligence experts, as Israel is not known to have expressly approved the leaking of its intelligence. (Israeli officials would not confirm they were the source, the Times said.)

The development is nevertheless a concern, in part due to Russia's close alliance with Syria, as well as Iran, which has called for the destruction of the Jewish state.

"This will cause … profound damage," says Mark Lowenthal, a former assistant director at the Central Intelligence Agency. "If [the source] had to be one of the Arab states, it would have been bad, but this is worse that it's Israel.

"The Israelis have such a strong intelligence capacity, and we have so much to gain from them."

Third-party intelligence from the Israelis — as with many of the country's espionage "liaison relationships" abroad — is considered vitally important to the CIA, Lowenthal says.

Intelligence from Israeli spies has helped the U.S. determine Iran's progress on its nuclear aspirations, for example. And the Five Eyes alliance of the U.S., the U.K., Australia, Canada and New Zealand is one of the most comprehensive espionage coalitions known.

Unshakable bond?

In 1956, when the full text of Nikita Khrushchev's infamous "secret speech" denouncing Joseph Stalin fell into the hands of Israeli spies, they passed the leaked transcript to the CIA. The U.S. government published the document and Khruschev's words dramatically undercut Soviet policy, accelerating the demise of the U.S.S.R.

And 60 years later, Israel remains crucial to the U.S., both diplomatically and in terms of security.

Even as he denied the reports about a leak, national security adviser H.R. McMaster on Tuesday reaffirmed "America's unshakable bond with our closest ally in the Middle East."

White House Credibility

National security adviser H.R. McMaster speaks during a briefing at the White House on Tuesday. (Susan Walsh/Associated Press)

How rattled Israeli intelligence might be by the latest reports is the big question.

That January report about U.S. officials warning Israelis precisely of this kind of scenario was "mind-blowing" in itself, says Elizabeth Goitein, who co-directs the Brennan Center for Justice's Liberty and National Security Program.

"I don't know what's more extraordinary: the fact U.S. intelligence officials reportedly warned another country about disclosing sensitive intelligence to the incoming administration, or the fact that this warning appears to have been warranted."

Goitein still expects a robust intel-sharing program to continue between the U.S. and Israel, "but that doesn't mean we'll receive every shred of intelligence that we would have received" before the latest reports surfaced.

Robert Jervis, a political scientist and intelligence expert at Columbia University, shares those anxieties. He believes partner nations will become "more hesitant" to provide intelligence if the president displays a carelessness with classified material.

In dealing with Trump, Jervis says, the dilemma for agencies abroad is whether they now feel they can trust the U.S., as the country's president is privy to top-secret material.

"People in the intelligence agencies live by keeping their word," he says. "If the counterpart says, 'Don't share,' you don't share. It's the rules of the game. You break with that, even if it doesn't result in great harm, the trust is broken."

Foreign allies play role in domestic security

Coverage by espionage allies, as well as satellites and listening posts, give the U.S. a broader network of intelligence in regions that are culturally unfamiliar or difficult to reach. The Brits, for example, operate a "listening post" in Cyprus that shares intelligence with U.S. agents, Jervis notes.

"You could put three or four NSA ships in the Mediterranean, or you could pick up [surveillance] that Israel picks up for you," he says. "It's much less expensive."

Although Americans may not feel the repercussions of foreign intelligence, Jervis says the sharing of classified material is vital for domestic security, particularly when it comes to the threat of ISIS.

"The whole domestic security arrangement depends on good intelligence about who's coming at us, how they're coming at us, and where they're coming at us from," Jervis says. "You can't do that through metal detectors. You can't do that with profiling. We're blind and deaf. Our safety depends on good foreign intelligence."

Lowenthal, the former CIA executive, frames the president's foreign intelligence woes in a more reductive way.

"Intelligence is based on trust, not just commonality of interests. It's as simple as, 'If I give this to you, can you keep a secret?" he says. "The answer, in this case, is apparently not."