Iran on Monday dealt a new blow to a compromise offer from Russia on its nuclear programme, saying it would only consider such a deal if it acknowledged the Islamic republic's right to enrich uranium on Iranian soil.
"As we said before we want to have enrichment inside Iran... and any proposal which is based on this principle will be studied," government spokesman Gholam Hossein Elham told reporters.
"We are studying the Russian proposal based on this framework," he said. "The government will never give up its principles."
Moscow has suggested allowing Iran to conduct uranium enrichment in Russia, giving the country access to the nuclear fuel cycle while guaranteeing its nuclear program is for peaceful purposes.
The Russian proposal seeks to overcome the key sticking point in talks between Iran and the European Union over the programme, which the United States alleges is a cover for nuclear weapons development.
Elham's comments came after top nuclear negotiator Ali Larijani criticised the Russian proposal for having "serious problems."
"It is an idea, not a structured proposal, we don't see it as mature and it has serious problems," Larijani, the secretary of the country's Supreme National Security Council, said on state television.
Iran has denied it is seeking to build a nuclear bomb and says it is seeking only to produce electricity.
However, Larijani did not completely reject the Russian proposal.
"The (Russian) plan could be complementary and supporting, there are technological benefits, we have to examine them. It is not rigid and there is room for maneuver," he said.
Last week, another top national security official had appeared warm to the Russian proposal when he vowed that his country would study it carefully.
Supreme National Security Council member Javad Vaidi also told the ISNA agency on December 28 that the Russian proposal was based on the establishment of a "joint Iran-Russia company on Russian soil" for the enrichment of uranium.
However, he gave no indication of whether Iran was ready to drop its long-standing demand to enrich uranium on its own territory.
Russia enjoys close ties with Iran and is helping build the Bushehr nuclear power plant in southern Iran.
Iran's stance toward the Russian proposal will undoubtedly impact fresh talks with EU negotiators, which are set to resume on January 18 and are aimed at securing guarantees from Iran over Western concerns that Tehran seeks nuclear weapons.
The two sides have acknowledged that wide differences remain, with Iran insisting on its right to make nuclear fuel, and the West fearful that this could be used to manufacture atom bombs.
Larijani also expressed disappointment with recent talks with Britain, France and Germany, the parties leading the European negotiations.
"I had hoped the recent negotiations in Vienna would make more progress. I don't have a negative view, but we hope future steps will be taken more seriously and carefully," he said.
Larijani warned the West, which has threatened to push for the United Nations Security Council to impose sanctions, about taking strong action.
"If they act harshly, our behaviour will change. We have prepared scenarios and they cannot checkmate us easily," he said.
Meanwhile, Iranian state television reported Sunday that the Islamic republic had developed technology to separate uranium from its ore, reinforcing its self-sufficiency in the nuclear fuel cycle.
The technology, known as a mixer-settler, is used to separate the uranium from the mined ore to produce concentrated uranium oxide - also known as yellowcake - which can then be converted and enriched in nuclear fuel work.