The United States should support Israel killing Supreme Leader Ali Khamenei and "any Iranian of any rank" who participates in the preparation of a dirty bomb, according to one national security expert.

Michael Rubin, from the American Enterprise Institute think tank, wrote that although Israel's air strikes hit Iran before it could build a nuclear weapon, the Islamic Republic possesses large stockpiles of highly enriched uranium that dirty bombs could spread to cause widespread contamination.

"Khamenei is 86-years-old and now among the last of his generation of revolutionary officials; he wants to see his life's work achieved in the next days," Rubin wrote in a Sunday opinion column.

"There is little doubt that the Islamic Republic has a huge stockpile of uranium, in quantities and with enrichment levels far beyond what it would need for either a civilian nuclear energy program or medical purposes."

He added that "the goal of Iran's arsenal was always Israel's eradication."

Reuters reported Sunday that Trump vetoed an Israeli plan in recent days to kill Khamenei, and that top U.S. officials have been in constant communications with Israeli officials in the days since Tel Aviv launched a massive attack on Iran in a bid to halt its nuclear program.

Rubin, though, said the U.S. erred when it defined Iran's nuclear weapons capability as achieving 20 kilograms of 90% enriched uranium.

"Essentially, the intelligence community twisted its intelligence by using an arbitrary definition to deny Iranian goals and achievements," Rubin wrote.

"In reality, nuclear weapons can have great destruction at enrichment levels below 90%. The uranium in the ‘Little Boy' bomb dropped on Hiroshima, for example, had an average enrichment level of 80%, a level Iranian scientists had already achieved."

That makes a desperate Khamenei all the more dangerous.

"If Khamenei wants to make good on his threats, his next step will likely be to hit Israel with dirty bombs to spread radiation over large sections of its urban areas," Rubin wrote.

"The same strategy would satisfy Khamenei's desire for revenge against Arab states that help Israel."

According to Rubin, the Israelis have "no choice but to kill Supreme Leader Ali Khamenei."

"The Iranian ayatollah is a legitimate target due to his constitutional role as commander-in-'chief of the Iranian military,'" Rubin wrote.

Even if it does not participate directly, the U.S. should assist Israel with its intelligence and information operations, according to Rubin.

"President Donald Trump should use his bully pulpit to make clear that any Iranian of any rank who participates in preparation of a dirty bomb will not only be killed, but their property will also be destroyed," he wrote.

"If the U.S. intelligence community has penetrated sufficiently, it should even announce the names of those it is watching and to which it is speaking in order to deter their action."