Atta Kenare / AFP / Getty Images Iranians hold a portrait of assassinated nuclear scientist Mostafa Ahmadi-Roshan during his funeral after the Friday prayers outside Tehran university on January 13, 2012.

Israel’s intelligence services have scaled back covert operations inside Iran, ratcheting down by “dozens of percent” in recent months secret efforts to disable or delay the enemy state’s nuclear program, senior Israeli security officials tell TIME. The reduction runs across a wide spectrum of operations, cutting back not only alleged high-profile missions such as assassinations and detonations at Iranian missile bases, but also efforts to gather firsthand on-the-ground intelligence and recruit spies inside the Iranian program, according to the officials.

The new hesitancy has caused “increasing dissatisfaction” inside Mossad, Israel’s overseas spy agency, says one official. Another senior security officer attributes the reluctance to Prime Minister Benjamin Netanyahu, who the official describes as worried about the consequences of a covert operation being discovered or going awry. Netanyahu was Prime Minister in 1997 when a Mossad attempt to assassinate senior Hamas official Khaled Meshaal in Amman Jordan ended in fiasco. Two Mossad operatives were captured after applying a poison to Meshaal’s skin, and returned to Israel only after Netanyahu ordered the release of the antidote. The Prime Minister also was forced to release Hamas’ spiritual leader Sheik Ahmed Yassin from an Israeli prison, dramatically boosting the fortunes of the religious militant movement.

“Bibi is traumatized from the Meshaal incident,” the official says. “He is afraid of another failure, that something will blow up in his face.”

Iranian intelligence already has cracked one cell trained and equipped by Mossad, Western intelligence officials earlier confirmed to TIME. The detailed confession on Iranian state television last year by Majid Jamali Fashid for the January 2010 assassination by motorcycle bomb of nuclear scientist Massoud Ali Mohmmadi was genuine, those officials said, blaming a third country for exposing the cell.

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In that case, the public damage to Israel was circumscribed by the limits of Iran’s credibility: Officials in Tehran routinely blame setbacks of all stripes on the “Zionists” and “global arrogance,” their labels for Israel and the United States. But that could change if the Islamic Republic produced a captured Israeli national or other direct evidence – something on the lines of the closed circuit video footage and false passports that recorded the presence of Mossad agents in the Dubai hotel where Hamas arms runner Mahmoud al-Mabhouh was found dead in his room in January 2010. Difficult-to-deny evidence of Israeli involvement trickled out for weeks; Netanyahu was Prime Minister then as well.

The stakes are higher now. With the Iranian issue at the forefront of the international agenda, a similar embarrassment could undo the impressive global front Washington has assembled against the mullahs — perhaps by allowing Iran to cast itself as victim, or simply by recasting the nuclear issue itself, from one of overarching global concern into a contest confined to a pair of longtime enemies.

Some warn that the assassinations already run that risk. After the most recent killing, of nuclear scientist Mostafa Ahmadi-Roshan in January, the United States “categorically” denied involvement in the death and issued a condemnation. Western intelligence officials say he was at least the third Iranian scientist killed by Mossad operatives, who lately are running short of new targets, according to Israeli officials.

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“It undercuts the consensus, the international consensus on sanctions,” says Mark Fitzpatrick, a former State Department nuclear proliferation specialist who opposes the assassinations.

The covert campaign also invites retribution from Iran’s own far-reaching underground. In the space of just days last month, alleged Iranian plots against Israeli targets in Thailand, Azerbaijan, Singapore and Georgia were announced as thwarted, and Indian officials blamed Iran for a nearly fatal attack that went forward in New Delhi. The wife of an Israeli diplomat was injured by a magnetic bomb attached to her car by a passing motorcyclist, the precise method Israeli agents are alleged to have used repeatedly on the crowded streets of Tehran.

But scaling back covert operations against Iran also carries costs, especially as Iran hurries to disperse its centrifuges, some into facilities deep underground. Quoting an intelligence finding, one Israeli official says Iran itself estimates that sabotage to date has set back its centrifuge program by two full years. The computer virus known as Stuxnet — a joint effort by intelligence services in Israel and a European nation, Western intelligence officials say — is only the best known of a series of efforts to slow the Iranian program, dating back years. That alleged effort involves a variety of governments besides Israel, involving equipment made to purposely malfunction after being tampered with before it physically entered Iran. The resulting setbacks prompted Iran to announce it would manufacture all components of its nuclear program itself – something outside experts are highly skeptical Tehran has the ability to actually do.

“Iran has said for some time that they’re self-sufficient, but that’s a bag of wind,” says Fitzpatrick, now at London’s International Institute for Strategic Studies. For example, Iranian President Mahmoud Ahmedinejad in February announced that Iran had perfected a far more efficient centrifuge — a “fourth-generation” machine, three levels beyond its original centrifuges, made from designs purchased from Pakistan’s A.Q. Khan. Fitzpatrick has his doubts. “They haven’t been able to get the second generation to work over the last ten years,” he says.

The alternative is importing equipment, which leaves the product vulnerable to continued tampering — especially in the shadowy markets of front companies where Iran has been forced by U.S. and international sanctions to do much of its business. It can be almost impossible to know whom you’re actually doing business with, a circumstance that favors Western intelligence agencies.

“The easiest way to sabotage is to introduce faulty parts into the inventory from abroad,” says Fitzpatrick.

Between assassination and silent sabotage lies another covert option: Very loud sabotage. Recent years have brought a series of mysterious explosions at complexes associated with Iran’s nuclear program. TIME has reported Western sources saying that Israel was responsible for the massive November blast at a Revolutionary Guard missile base outside Tehran, which by dumb luck also claimed the life of the godfather of Iran’s missile program.

But other blasts remain genuine mysteries. Weeks after a huge explosion darkened the sky over a uranium enrichment site in Isfahan, in central Iran, Israeli officials appeared eager to see what had actually happened. “I’m not sure what,” a retired senor intelligence official said two weeks afterward, then offered an analysis based on open-source satellite photos available to anyone with an internet connection.

— Aaron J. Klein contributed from Jerusalem

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