Iran has rejected the West's demand to send nuclear material out of the country but signalled flexibility on other aspects of its atomic activities.
Talks about Iran's nuclear programme, due to start in Geneva on Tuesday, will be the first since the election of Iranian President Hassan Rouhani, who has tried to improve relations with the West to pave a way for lifting economic sanctions.
Iranian Deputy Foreign Minister Abbas Araqchi's comments on Sunday may disappoint Western officials, who want Iran to ship out uranium enriched to a fissile concentration of 20%, a short technical step away from weapons-grade material.
However, Araqchi, who will join the talks in Switzerland, was less hardline about other areas of uranium enrichment, which Tehran says is for peaceful purposes but the West fears may be aimed at developing nuclear weapons capability.
"Of course we will negotiate regarding the form, amount, and various levels of [uranium] enrichment, but the shipping of materials out of the country is our red line," he was quoted as saying on state television's website.
In meetings over the last two years, Western negotiators have demanded that Iran suspend 20% enrichment, send some of its existing uranium stockpiles abroad and shutter the Fordow production site buried deep inside a mountain south of Tehran, where most higher-grade enrichment work is done.
In return, they offered to lift sanctions on trade in gold, precious metals and petrochemicals. Iran, which wants an easing of stringent oil and banking sanctions, dismissed that offer in meetings earlier this year. It says it is enriching uranium to 20% to fuel a medical research reactor in Tehran.
Araqchi did not specify the 20% material but his comments implied that Tehran might refuse at least this part of the powers' demands.
Since the Islamic Republic started producing 20% enriched uranium in 2010 it has produced more than the 240-250kg of uranium gas needed for a bomb, which Israel has suggested it sees as a "red line" for possible military action against Iran.
But Iran has kept its 20% stockpile below this figure by converting some of the gas into oxide powder for reactor fuel.
Iran says its nuclear programme is a project to generate electricity. The United States and its allies have imposed wide-ranging sanctions on Iran's banking, energy and shipping sectors to persuade it to curb the programme.
Iran has amassed stocks of low- and medium-enriched uranium gas—6.8 tonnes and 186kg respectively—that experts say would be enough for several bombs if processed further to weapons-grade material.
R Scott Kemp, an assistant professor of nuclear science and engineering at the Massachusetts Institute of Technology, wrote in an October 10 blog that merely capping Iran's nuclear programme is unlikely to provide enough confidence in the West.
"Some rollback of the programme, even if not done immediately, is really the only path to confidence and stability," Kemp wrote.
Israel, which has been lobbying world powers to keep up sanctions against the Iranians and has threatened preemptive military action if it deems diplomacy a dead end, demands a total removal of Tehran's enrichment uranium stockpiles along with a dismantling of its enrichment facilities.
Asked why the Iranians should be stripped of all their uranium if they were also to be stripped of the means to purify them to bomb-grade, Israeli officials say they fear Tehran may have covert enrichment plants unknown to world powers.
One senior Israeli official, speaking to Reuters on condition of anonymity, sought to turn this logic against Iran, saying: "Since they won't have enrichment capabilities and won't have anything to do with the stockpiles, why should they keep them? Why are they insisting? That in itself is suspicious." - Reuters