Iranian news agencies reported an explosion Monday in the central Iranian city of Isfahan, which hosts a nuclear research facility, but on Tuesday the provincial deputy governor denied there had been any explosion.

It was never made clear where the reported blast occurred, although there was nothing to link it to the research facility attached to the city's university.

But the explosion-that-never-was, the Iranians say, underlines how the country is being spooked by covert operations against its nuclear program by U.S. and Israeli intelligence services.

The Isfahan episode occurred just more than two weeks after a massive explosion at a ballistic missile base near Tehran killed the architect of Iran's strategic missile program, Maj. Gen. Hassan Tehrani Moghaddam.

It also came hard on the heels of an Iranian announcement that security authorities had captured a dozen "CIA spies" targeting the nuclear program. That followed reports earlier in the week that other alleged CIA operatives had been rounded up in May.

None of the reports have been verified and the CIA declined comment, although agency officials had in recent days admitted -- unusually -- that bungled operations in Beirut against Hezbollah, Iran's key proxy in Lebanon, had led to the capture of a dozen Lebanese informants.

"The U.S. and Zionist regime's espionage apparatuses were trying to damage Iran both from outside and inside with a heavy blow, using regional intelligence services," declared Parviz Sorouri of the Iranian Parliament's national security committee.

Iranian officials say the Nov. 12 blast at the al-Ghadir base, a storage and testing area for Shehab-3 ballistic missiles, was an accident that occurred during the testing of a new missile.

Israel's foreign intelligence service, the Mossad, has been widely seen as responsible for the explosion as part of its clandestine campaign to sabotage Iran's nuclear program. However, Iranian Parliament Speaker Ali Larijani denied that Israel was involved.

Israel has made no direct comment on this. However, Intelligence Minister Dan Meridor observed elliptically after Monday's report of an explosion that in dealing with the Iranian threat "there are countries that impose economic sanctions and there are countries who act in other ways."

Meantime, Brig. Gen. Ithai Baron, head of the research directorate of Israel's Military Intelligence, told Parliament's Foreign Affairs and Defense Committee Monday that al-Ghadir explosion could delay Tehran's drive to produce intermediate-range ballistic missiles capable of reaching the Jewish state.

But he stressed, "We must emphasize that Iran has other development tracks in addition to that facility."

The al-Ghadir blast followed the assassination and defection of several Iranian nuclear scientists and a formidable cyberwarfare assault, suspected to be run by Israeli intelligence, that damaged Iran's nuclear program.

Iranian officials have copiously accused Israel and the United States of covert operations but insist that the al-Ghadir blast was an accident.

In recent days, Iran's leading generals have been falling over themselves to warn the Americans and Israelis of the dire and terrible retaliation they face if the Islamic Republic is attacked, as Israel has threatened to do.

Veteran Middle East analyst Mahan Abedin sought to explain this paradox by observing: "Mischievous Israeli posturing notwithstanding, there is no evidence or credible information at this stage to suggest that the explosion at the al-Ghadir base was anything but an accident caused by an important experiment involving ballistic missiles and high explosives.

"But assuming the explosion was the result of sabotage, senior Iranian officials have two overriding reasons to insist on an accidental cause.

"In the very short term, an admission that sabotage is the cause runs the risk of inflaming public opinion with the resulting overwhelming demand for immediate retaliation," Abedin noted.

"For various reasons -- not least the desire to avoid escalation -- Iranian leaders are not overly keen to respond to Israeli and American provocations which they view as a trap.

"At a deeper level, this remarkable forbearance in the face of seemingly intolerable provocations is the result of Iranian leaders' strategic calculus.

"Iran's leaders long ago concluded that enormous pressures -- including sabotage operations -- would be directed against the country to coerce the leadership to discontinue the nuclear program," Abedin observed in an Asia Times analysis.

"By refusing to retaliate against the country's enemies, Iranian leaders are sending yet another signal that they are committed to staying on the same strategic trajectory regardless of the costs."