BUSHEHR, Iran -- Iran and Russia agreed in principle Sunday to establish a joint uranium enrichment venture, a breakthrough in talks on a U.S.-backed Kremlin proposal aimed at easing concerns that Tehran wants to build nuclear weapons.
But further negotiations on the details lay ahead, and it was not known whether Iran will entirely give up enrichment at home, a top demand of the West.
The deal -- announced by the two countries' top nuclear chiefs after a visit to a Russian-built nuclear plant in southern Iran -- could deflect any move by the U.N. nuclear watchdog agency at its March 6 meeting to recommend the Security Council consider action on Iran.
Iran's deputy nuclear chief, Mohammad Saeedi, warned that the deal would be off if the International Atomic Energy Agency refers Iran to the Security Council, a step that could lead to economic or political sanctions.
"If talk of referral is raised, then all ways will be blocked," Saeedi said.
Russian participation in the project is aimed at ensuring that no enriched material is secretly diverted to a weapons program.
The United States accuses Iran of seeking to develop nuclear weapons but has backed the proposal if it means enrichment would take place entirely in Russia. Iran denies any intention to build weapons, saying it aims only to produce nuclear energy.
U.S. National Security Adviser Stephen Hadley was cautious about the deal.
"It's too soon to say," he said on CNN's "Late Edition." "In any of these arrangements, the devil is in the details. We'll just have to see what emerges.
It appeared that the issue of Iran's domestic enrichment was still unresolved between Tehran and Moscow.
The head of Iran's nuclear agency, Vice President Gholamreza Aghazadeh, and his Russian counterpart Sergei Kiriyenko avoided addressing the issue in a short news conference announcing the agreement Sunday. The two spoke in Bushehr, the site of Iran's first reactor, built with Russian aid and due to be inaugurated this year.
Russia, a top ally of Iran, has been pressing Tehran to have all its uranium enrichment take place on Russian soil -- and had made the deal contingent on Iran's formally calling off its domestic enrichment program, based in the central city of Natanz.
But Iran rejected that linkage, insisting on its right to carry out enrichment.
Enrichment is a key process that can determine the direction of a nuclear program. Uranium enriched to a low level produces fuel for a nuclear reactor, while higher enrichment produces the material needed for a warhead.
In tough negotiations over past weeks, Iran has said the location of enrichment and the degree of each side's role in the joint venture had to be worked out.
"We reached a basic agreement on the creation of a joint venture. How this is going to be created needs a lot of discussion, and talks will continue in Moscow in the next few days," Aghazadeh said.
Kiriyenko said the Russian-proposed joint venture, if successful, would help achieve a peaceful solution to the standoff over Iran's nuclear program.
"Russia has made its proposal to Iran for a joint uranium enrichment venture on Russian territory. If this proposal is carried out, a good opportunity will be created. Negotiations about the creation of this venture are continuing actively," he said.
He suggested Russia would prevent any move by the IAEA at its March 6 session to put Iran before the Security Council.
"There is a possibility that Iran's nuclear activities can be resolved within the IAEA," he said. "Since there is such a possibility, we have to take all necessary measures to achieve this."
Earlier this month, the IAEA board of governors -- including Russia -- voted to report Iran to the Security Council. But it did not ask the council to take any immediate action, putting off any possible move for sanctions.
Saeedi, Aghazadeh's deputy, said European countries and China could become involved in the joint venture, with Russia playing a leading role.
Meanwhile, Aghazadeh said the Bushehr nuclear power plant was "90 percent complete" and that Iran will build two more 1,000-megawatt nuclear power plants at the site.
Iran will issue tenders for the construction within a month, he said, suggesting Russia would be a favorite to build the plants.
More than three years of IAEA probing have failed to produce concrete evidence of any Iranian nuclear weapons program. But the agency discovered suspicious activity, including plutonium experiments and long-secret efforts to develop enriched uranium.
An IAEA team was in Tehran on Sunday, and Iranian foreign ministry spokesman Hamid Reza Asefi said Iranian officials would discuss with the team a secret nuclear project that U.S. intelligence has claimed was linked to warhead design.
Associated Press reporter Vladimir Isachenkov in Moscow contributed to this report.