The differences in the assessments of Israeli and American intelligence agencies regarding Iran's nuclear program mainly stem from the question of when Iran will succeed in building a basic atomic bomb and how much time it will take Iran's scientists and engineers to place a nuclear warhead atop a ballistic missile.
Building a reliable nuclear weapon requires efforts in three separate tracks: The first, enriching uranium to 90% purity level. Fissile material can also be produced from plutonium that is made from fuel rods used in heavy water reactors. The Iranians are pursuing this track, but it will take them some two years to create fissile material. However, by continuing to enrich uranium the Iranians will be able to produce fissile material by the end of the year.
The second track is the construction of a prototype of an atomic bomb. The problem is that such a bomb, in its most basic form, is very large and cannot be placed atop a ballistic missile or even attached to the wing of a fighter jet. Dropping the bomb on the enemy requires a strategic bomber that only the US, Russia and Britain currently have – or, alternately, a container that can be smuggled aboard a ship. But in order for a country to possess the complete military nuclear capability Iran seeks, it must be able to reduce the size of the bomb so it will be able to fit into the warhead of a ballistic missile with a diameter of a little more than one meter. The third track, therefore, is the reduction in size of the bomb.
There is no argument between Israel and the US regarding Iran's ability to produce fissile material for a bomb by the end of this year, or by February 2014 at the latest. President Obama's estimation that Iran will be able to build a nuclear weapon only in a "year or more" is based on the American intelligence assessment that it will take the Islamic Republic at least another year to build a basic prototype of a nuclear bomb and reduce its size so it will fit into a missile.
Obama's interview with AP
Israeli experts consider this assessment to be a bit far-fetched for the simple reason that the production of a basic atomic bomb and reducing its size is a process that is more difficult to monitor than uranium enrichment, which is conducted in huge plants the size of several soccer fields and is supervised by inspectors of the International Atomic Energy Agency. IAEA inspectors have been denied access to sites where Iran is suspected to be carrying out experiments related to the development of nuclear bombs. These sites are relatively small and easy to conceal, as are the sites where Iran is developing nuclear warheads.
The Iranians are denying access to these sites for fear that the inspectors will find radioactive material that is still traceable and facilities in which controlled-detonations are being carried out.
It is safe to assume that Israeli intelligence officials are telling their American counterparts: "Both you and we do not know how far along Iran is in the development of a bomb and if they have already managed to miniaturize it. According to testimonies revealed in some WikiLeaks documents, Iranian scientists received computer programs to design a nuclear warhead for a missile based on the Chinese model and that intensive work has also been conducted to design an atomic bomb, but we mustn't assume that what we know for certain makes up the entire picture. It is very possible that the Iranian nuclear program has advanced much further and that as soon as they have 28 kilos of fissile material they will already have the capability to build a basic bomb and place it inside the warhead of a ballistic missile."