These are some highly simplified drawings of advanced nuclear warheads that probably employ ablation implosion, also known as the Teller-Ulam design. What is left out, though, are a few key details. Not mentioned at all is the now almost mythical fogbank, for example, or any x-ray mirrors or other components which may be crucial to efficient fusion. The nature and roles of the filler, foam, and plasma are still not well known. Because the description of how the x-rays do their work is so vague, one can expect that the information about x-ray tuning are similarly vague.
There is an outside chance that some wildcard technology has remained totally classified to date. There may be a tiny enabling factor that has been carefully hidden over the years. Even if every detail of the construction is known, the ability to create a fusion boosted fission bomb is far harder than a simple fission device such as a gun-type warhead. This is because the modeling, metallurgy, and design are all extremely complex. In addition, simple fission bombs are not ideal primary stages for staged fission-fusion-fission bombs. They may not be hot enough, or small enough. Therefore, there is little risk of a nation such as Iran developing advanced warheads of this type any time soon. Certainly not without some significant testing. And if they do build warheads but never test them, can they be considered a viable threat? In any event, a crude 10 Kt gun-type warhead would be an easy first step for Iran.
For a counter-argument, suggesting that these smaller semi-nuclear states could build a thermonuclear bomb, visit Global Security.