JERUSALEM - Mishaps at Iranian nuclear facilities and weapons sites may be part of a covert organized attack on Iran's nuclear weapons program, according to Western intelligence officials.

An explosion last week outside Iran's third largest city, Isfahan, is thought to be the most recent strike, though details on the intended target are still unclear. A sprawling military base and nuclear facilities are outside Isfahan, and intelligence officials across the Middle East said there was strong evidence that the explosion had done "significant structural damage."

"We have seen enough evidence to know something has happened, but we are unclear on what at this point," said one Western diplomat whose work includes the region, and who spoke on the condition of anonymity because of the sensitivity of the issue. "There are a number of parties interested in sabotaging and weakening the Iranian regime."

The second such incident in as many weeks cast doubt on Iranian claims that these were "accidents" and "coincidences," and it set off speculation of a coordinated attack by Israel, whose officials long have threatened a strike against Iran's nuclear program. Israeli officials denied direct involvement, but the growing number of mysterious or unexplained blasts and deaths has many suspecting an official program of sabotage.

Israeli newspapers declared last week that Israel's war with Iran already had begun, but that the Jewish state, rather than launch massive airstrikes, had decided on covert action in cooperation with other groups. Statements by current and former Israeli officials were being parsed for clues but did little to clarify the issue.

"There aren't many coincidences, and when there are so many events there is probably some sort of guiding hand, though perhaps it's the hand of God," said Israel's former head of internal security, Maj. Gen. Giora Eiland.

The sites in Iran that have been damaged would appear to be significant targets.

The nuclear facilities in Isfahan are involved in converting yellowcake into uranium hexafluoride gas, a key step in producing enriched uranium. Two weeks ago, an explosion at a missile base west of Tehran killed more than 20 members of Iran's Revolutionary Guard Corps, including Cmdr. Hassan Moghaddam, described as the architect of Iran's missile program.

Those, experts note, are only the most recent cases. A half-dozen other mysterious explosions and accidental deaths have been reported over the last two years. They include an October 2010 explosion at a military base housing Shahab-3 missiles near the city of Khorramabad and the July 2011 assassination of nuclear scientist Dariush Rezaei in Tehran.

Stuxnet, a computer virus that wreaked havoc on the centrifuges in Iran's uranium enrichment facilities, has rewritten the playbook for international cyber-terrorism.

Experts say the incidents have set back Iran's nuclear ambitions while allowing the country's foes to deny involvement.

"On the one hand, you have Iran's nuclear weapons program slowed down. On the other hand, you don't have to take responsibility for doing anything, and Iran will continue to deny them and call them accidents," said one Western intelligence official, who wasn't authorized to speak publicly on the issue.

He added that the method "served both parties for the time being" but that it could "only continue for so long."