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 to produce nuclear energy.

It appeared that the issue of Iran’s domestic enrichment was still unresolved between Tehran and Moscow.