Recent intelligence reports accuse North Korea of secretly helping Iran develop its nuclear programme, raising fresh concerns about Pyongyang's nuclear proliferation and Tehran's atomic intentions.
The United States and the European Union fear Iran is using its nuclear energy programme as a front to develop nuclear weapons and have called on Iran to cease all sensitive atomic work.
Tehran says its programme is peaceful and refuses to give up its sovereign right to a full atomic programme.
"In the late 1990s, cooperation began between the two countries, which focused on nuclear (research and development)," said an intelligence report obtained from a non-US diplomat.
"There has been a significant improvement in relations between Iran and North Korea over the past few months," the report said. A recent example is what the three-page report described as a "special secret course to provide technological and practical information to outstanding students."
Among the lecturers are senior North Korean scientists and atomic technicians, it said. "This nuclear cooperation between the two countries has apparently increased significantly during the past year as seen in the arrival of an academic delegation from North Korea in Iran and the existence of this special course," it said.
The secret masters level course at Tehran's Polytechnic University covered "dual use" nuclear technology that could be applied to civilian or military applications, the report said.
"It seems Iran is taking another step to promote its military nuclear project by exploiting North Korea's extensive technological information in the nuclear sphere," it said. A senior Iranian official who was shown the report did not respond to several requests for a comment.
A nuclear expert who was involved in the International Atomic Energy Agency's (IAEA) investigation of Iran's atomic programme said there was no way the IAEA would get access to this kind of information but he said it was credible.