Iran has reached the "technical ability" to produce highly enriched uranium, an essential step on the way to making an atomic bomb, the U.K. newspaper The Guardian quoted on Thursdays a diplomatic cable leaked by the WikiLeaks site.

Citing a diplomatic cable from April 2009, The Guardian quotes a U.S. official at a meeting of international nuclear experts in Vienna as indicating that "Iran had now demonstrated centrifuge operations such that it had the technical ability to produce highly enriched uranium (HEU) if it so chose".

The U.K. paper added that the U.S. official added at the time that such advancement was an essential step towards building a nuclear bomb, according to a leaked US diplomatic cable.

The report of Iran's alleged nuclear advancement came as Iran's envoy to the United Nation's atomic agency said that the country would be able to continue enriching uranium even if its nuclear facilities suffered a military strike.

Speaking with reported, Ali Asghar Soltanieh told reporters that Iran was dealing "with a very serious threat and so we have had to take measures to protect our facilities. We have provided for another facility in Fardo near Qom,"

It is, so to speak, a reserve facility, so that if a site is attacked, we can continue the enrichment process," he said.

The comment by the Iran nuclear official came after Tehran denied reports alleging that it planned to revive a nuclear fuel swap proposal, saying it was, however, ready to discuss it in talks with world powers on Friday.

Expectations of any breakthrough in an eight-year-old stand-off over Iran's nuclear ambitions were low ahead of a second round of negotiations between Iran and six powers in the Turkish city of Istanbul on Friday and Saturday.

The six dealing with Iran via European Union foreign policy chief Catherine Ashton are the United States, Britain, France, Russia, China and Germany, and there were resurfacing signs of differences within the group that Iran has sought to exploit.

Speaking on the eve of the talks, Russian Foreign Minister Sergei Lavrov said they should look at prospects for relieving punitive sanctions on Tehran. He criticized the United States and European Union for imposing sanctions unilaterally that went beyond those agreed by the UN Security Council last June.

There is international concern that Iran's declared civilian nuclear energy program is a cover for pursuit of atom bombs. Escalating economic sanctions have been slapped on Tehran over its refusal to curb nuclear work and make it more transparent.