North Korea has supplied Iran with a computer programme that could help it build nuclear weapons, a German newspaper said.
The Sueddeutsche Zeitung said North Korea had delivered software, originally developed in the United States, that could simulate neutron flows.
These calculations are linked to chain reactions and are vital in the construction of reactors as well as the development of nuclear explosives.
Iran could gain important knowledge of how to construct nuclear weapons using this programme, reported the newspaper citing intelligence sources.
This could add to Western suspicions about Iran's disputed nuclear activities and its links with North Korea, a secretive Asian state whose pursuit of nuclear weapons worries the world.
A confidential U.N. report earlier this year said North Korea and Iran appeared to have been regularly exchanging ballistic missile technology in violation of U.N. sanctions.
Iran rejects Western accusations it is seeking to develop atomic arms but its refusal to halt sensitive work has drawn gradually tightening U.N. and Western sanctions since 2006.
The Sueddeutsche said the programme, called Monte Carlo N-Particle Extended, or MCNPX 2.6.0., was used widely for civilian purposes but is subject to strict export controls because it can also be used to develop atomic weapons.
It is unclear how North Korea got hold of the software.
The deal could be part of a comprehensive cooperation between the two states for which Iran has paid more than $100 million, said the Sueddeutsche.
The paper also said a delegation from North Korea travelled to Iran in February to train 20 employees of the defence ministry in the software.
The U.N. nuclear watchdog has voiced growing concern in the last year about possible military links to Tehran's nuclear programme, saying it has received new information adding to those concerns.
"More information is coming and we are assessing it," Yukiya Amano, director general of the International Atomic Energy Agency said last week.
The IAEA has been investigating for several years Western intelligence reports indicating Iran has coordinated efforts to process uranium, test explosives at high altitude and revamp a ballistic missile cone so it could take a atom warhead.
Iran rejects the allegations as forged and baseless.