ISTANBUL (Reuters) - Iran warned it would go on enriching uranium if it came under attack as its negotiators prepared for talks with six world powers on Friday (today) aimed at defusing a crisis over Irans disputed nuclear programme. Expectations of any breakthrough in an eight-year-old stand-off over Irans nuclear ambitions were low ahead of a second round of negotiations between Iran and the powers in the Turkish city of Istanbul on Friday and Saturday. Iranian negotiators told Reuters they had no fresh offer to make for a nuclear fuel swap but they were ready to discuss a deal based on terms offered last year, which were rejected then as being too little, too late. A fuel swap deal, under which Iran would part with some of its low enriched uranium (LEU) in exchange for fuel specially processed to run a Tehran reactor producing medical isotopes, would build confidence but not resolve core disputes. Any accord is likely to hinge on persuading Iran to hand over most of its LEU stockpile to dispel fears that it was retaining enough of the material to develop a nuclear bomb by enriching it to a very high level of fissile purity. On the eve of the talks, speaking in Moscow, Ali Asghar Soltanieh, Irans envoy to the International Atomic Energy Agency (IAEA), struck a defiant stance, saying enrichment would continue even if nuclear facilities were attacked. We have provided for another facility in Fordow near Qom, Soltanieh said. It is, so to speak, a reserve facility, so that if a site is attacked, we can continue the enrichment process. Irans main enrichment plant is in Natanz. Fordow, a much smaller site that Tehran did not reveal to IAEA inspectors for over two years, is under construction inside a mountain bunker. In Istanbul, Russian Foreign Minister Sergei Lavrov said the talks should address prospects for easing economic sanctions on Tehran if it is more forthcoming with IAEA inspectors. Soltanieh said Iran was ready to discuss a swap on the basis of a deal brokered in May last year with Brazil and Turkey. We already made the maximum, historical concession with the Tehran Declaration, and that remains on the table, he said. Ali Baqeri, a deputy to Iranian nuclear negotiator Saeed Jalili, rejected a report by Saudi-owned Al Arabiya TV news channel that Iran would propose a revised version of the deal. Another Iranian official said: There is no new proposal. Signaling determination to keep up pressure on Iran, US Secretary of State Hillary Clinton told the US television network ABC the Obama administration may propose new unilateral sanctions on Iran, one of the worlds largest oil exporters. But Lavrov said unilateral sanctions were spoilers and the talks in Istanbul should look at ways of rolling back sanctions. We explained to our partners in the United States and the European Union what we think about unilateral sanctions and we hope they have heard us. It is counterproductive to continuing our common efforts to resolve the Iranian nuclear issue. Meanwhile, the United States said it wanted this weeks talks between major powers and Iran to lead to a meaningful and practical process that addressed core concerns about Irans nuclear programme. US Undersecretary of State William Burns will lead the US delegation at the talks, which begin on Thursday in Istanbul.