VIENNA It is one of the West's biggest nuclear proliferation nightmares -- that increasingly isolated Iran and North Korea might covertly trade know-how, material or technology that could be put to developing atomic bombs.
"Such a relationship would be logical and beneficial to both North Korea and Iran," said Mark Hibbs, an expert of the Carnegie Endowment for International Peace.
Last year, a U.N. report suggested that impoverished, reclusive North Korea might have supplied Iran as well as Syria and Myanmar with banned atomic technology.
In what could be a sign of this, a German newspaper last month reported that North Korea had provided Iran with a computer program as part of intensified cooperation that could help the Islamic state build nuclear weapons.
"There are reports and rumors, which governments and the IAEA (the U.N. International Atomic Energy Agency) have not denied, indicating that there may be a track record of bilateral nuclear cooperation between North Korea and Iran," Hibbs said.
But while this could make sense for two states facing tightening sanctions -- and potentially earn Pyongyang some badly needed funds -- the extent and nature of any such dealings, if they take place at all, remain shrouded in mystery.
"It seems to be very difficult to sort out what the relationship in the nuclear world between DPRK (North Korea) and Iran is. We just simply do not know," prominent nuclear scientist Siegfried Hecker said.
This was in contrast to missile cooperation between the two countries, where North Korea has helped Iran both with the weapons and in building related factories, he said.
Hecker, who has often visited the east Asian state, said possible Tehran-Pyongyang atomic technology transfers would be a major concern for everyone dedicated to preventing the proliferation of nuclear weapons.
Iran's nuclear program is based on uranium enrichment, activity which can have both civilian and military purposes.
North Korea has twice tested plutonium-based nuclear devices, drawing international condemnation, although it last year revealed the existence also of a uranium enrichment site, potentially giving it a second pathway to bombs.
"They complement each other so well (in terms of their expertise). There is just a lot of synergy in how they would be able to exchange capabilities," Hecker said at a seminar for diplomats in Vienna, the IAEA's headquarters, this month.
Citing Western intelligence sources, the Munich newspaper Sueddeutsche Zeitung said in August that North Korea had this year delivered software, originally developed in the United States, that could simulate neutron flows.
Such calculations, which can help scientists identify self-sustaining chain reactions, are vital in the construction of reactors and also in the development of nuclear explosives.
With the help of the program, Iran could gain important knowledge of how to assemble nuclear weapons, the paper said.
WESTERN INTELLIGENCE HUNT
There has been no public confirmation or denial of the report in the West. But Hecker did not rule it out, saying Pyongyang had demonstrated experience in this field.
He said North Korea must have some "nuclear code capabilities" which they would have been able to assess in comparison with the result of an atomic test.
"So to some extent they have had an opportunity to verify or check their codes," Hecker said. "Iran has not had a chance to do that. So exchanging that type of information ... you could see as being very useful."
North Korea tested nuclear devices in 2006 and 2009, but still has not shown it has a working nuclear bomb.
Proliferation experts have said the country has enough fissile material for up to 10 nuclear weapons. But they don't believe Pyongyang is yet capable of miniaturizing the material to fit into the cone of a missile.
While North Korea has made no secret of its nuclear weapons ambitions, Iran denies Western allegations that it is covertly seeking to develop an atomic arms capability.
The Islamic Republic says its nuclear program is for purposes of electricity generation, but its refusal to halt uranium enrichment and its stonewalling of a U.N. nuclear watchdog probe have stoked suspicions abroad.
The Vienna-based U.N. nuclear watchdog said this month in a report that it was "increasingly concerned" about possible work in Iran to develop a nuclear missile.
For several years, the IAEA has been investigating Western intelligence reports indicating Iran has coordinated efforts to process uranium, test high explosives and revamp a ballistic missile cone to accommodate a nuclear warhead.
Iran says the allegations are baseless and forged.
In a separate report on North Korea, from which its inspectors were expelled in 2009, the IAEA suggested past nuclear-related ties with Syria and Libya, but it made no mention of Iran.
Proliferation expert Mark Fitzpatrick said it would not be hard for Tehran and Pyongyang to put "well-practiced trade deals and transfer routes" from their ballistic missile cooperation to use also in the nuclear field. Still, there were few signs of any nuclear cooperation between the two countries.
"It is not for lack of looking. Western intelligence agencies are intensively targeting nuclear acquisition efforts by Iran and North Korea," Fitzpatrick, a former senior U.S. State Department official, said.
"Yet finding nuclear weapons-related trade is akin to the proverbial needle in a haystack. In the vastness of ocean and sky routes, most forms of nuclear-related cargo are so minute as to be almost undetectable."
Hibbs said any nuclear dealings with North Korea would pose risks for Iran: "Were this traffic to be confirmed, that would deepen the suspicion that Iran is involved in nuclear activities which are clandestine and military in nature."
(Editing by Mark Heinrich)