TEL AVIV, Israel -- Israeli intelligence officials now estimate that Iran won’t be able to build a nuclear weapon before 2015 or 2016, pushing back by several years previous assessments of Iran’s nuclear ambitions.

Intelligence briefings given to McClatchy over the last two months have confirmed that various officials across Israel’s military and political echelons now think it’s unrealistic that Iran could develop a nuclear weapons arsenal before 2015. Others pushed the date back even further, to the winter of 2016.

"Previous assessments were built on a set of data that has since shifted," said one Israeli intelligence officer, who spoke to McClatchy only on the condition that he not be identified. He said that in addition to a series of "mishaps" that interrupted work at Iran’s nuclear facilities, Iranian officials appeared to have slowed the program on their own.

"We can’t attribute the delays in Iran’s nuclear program to accidents and sabotage alone," he said. "There has not been the run towards a nuclear bomb that some people feared. There is a deliberate slowing on their end."

Reports that Iran’s nuclear facility at Fordow had been damaged in a nuclear explosion were still being investigated Monday, Israeli officials said. Satellite imagery shared with McClatchy showed that new fortifications had been built around the perimeter of the facility.

"This is already Iran’s most heavily fortified facility," said another intelligence officer, based in Israel’s central command. "The new construction we are seeing here is meant to prevent access to the facility through land routes."

He speculated that Iran had taken special care to protect its facilities in Fordow because it was a "highly attractive target for an attack."

"Despite repeated efforts by Iran to reinforce and protect their nuclear facilities, there have been accidents that some call sabotage that may have been carried out by a number of interested parties," he said, listing Iranian dissident groups that he said would try to attack Iranian military and nuclear facilities. "One way or another, Iran has been forced to slow down."

Writing in Israel’s Hebrew-language daily newspaper Yediot Ahronot, military correspondent Alex Fishman said, "Officials responsible for assessing the state of the Iranian nuclear program, both in the West and in the International Atomic Energy Agency, believe that while the Iranians have continued to pursue their nuclear program, they have been doing so cautiously and slowly, making sure not to cross the point of no return."

Fishman wrote that Israel’s allies in the West, including Europe and the United States, had been notified of the new calculations that Iran couldn’t possess nuclear weapons before 2015.

That assessment, he said, has been unpopular in Israel’s highest political echelons. Prime Minister Benjamin Netanyahu repeatedly has called 2013 a "decisive year" for Iran’s nuclear program. During his speech at the United Nations General Assembly in September, Netanyahu displayed a rudimentary bomb diagram to illustrate Iran’s progress toward a nuclear weapon.

"By next spring, at most next summer, at current enrichment rates, they will have finished the medium enrichment and moved on to the final stage,” Netanyahu said, laying out a timeline for the summer of 2013. “From there it’s only a few months, possibly a few weeks, before they get enough enriched uranium for the first bomb.”