Iran gained the "technical ability" to produce highly enriched uranium in sufficient quantities to make nuclear weapons in 2009, U.S. officials believe, marking a major achievement in Tehran's controversial atomic program, according to leaked diplomatic cables.

The U.S. assessment, released by WikiLeaks and published in Britain's Guardian newspaper, came after a meeting of international nuclear experts in Vienna in April 2009.

"Iran had now demonstrated centrifuge operations such that it had the technical ability to produce highly enriched uranium (HEU) if it so chose," U.S. Ambassador Gregory Schulte said.

Just weeks prior to the assessment, the International Atomic Energy Agency concluded Iran had enriched about one ton of uranium and needed just half a ton more to be ready to manufacture its first warhead.

The report came as Iran and six world powers sought common ground Friday at talks jeopardized by Tehran's refusal to discuss demands that it curb its nuclear activities.

The two sides sat down with no sign they were ready to budge from widely differing positions revealed after a first round of talks in Geneva last month.

While the six would like to kickstart talks focused at freezing Iran's uranium enrichment program, Tehran has repeatedly said "that" activity is not up for discussion. Instead, Iranian officials are pushing an agenda that covers just about everything except its nuclear program: global disarmament, Israel's suspected nuclear arsenal, and Tehran's concerns about U.S. military bases in Iraq and elsewhere in the region.

"We want to discuss the fundamental problems of global politics at Istanbul talks," said Iranian chief negotiator Saeed Jalili, while President Mahmoud Ahmadinejad suggested any push to restrict the meeting to Iran's nuclear program would fail.

"They employed all their might and tried hard to prevent Iran from going nuclear," Iranian state TV quoted Ahmadinejad as saying. "But Iran went nuclear and there will be no way back."

A diplomat familiar with the talks says the six powers will seek to nudge Iran toward acknowledging the need to reduce worries that the Islamic Republic might turn its enrichment program to making weapons. He asked for anonymity because the talks are closed.

Tehran denies such aspirations, insisting it wants only to make nuclear fuel. But concerns have grown because its uranium enrichment program could also make fissile warhead material, because of its nuclear secrecy and also because it refuses to cooperate with attempts to investigate suspicions that it ran experiments related to making nuclear weapons.

Iran came to the table warning that it was in no mood to compromise.

"Resolutions, sanctions, threats, computer virus nor even a military attack will stop uranium enrichment in Iran," Ali Asghar Soltanieh, Iran's envoy to the International Atomic Energy Agency, told Iranian state TV.

The enrichment program has sparked U.N. Security Council sanctions, been targeted by the Stuxnet malvare virus -- thought to have been manufactured by Israel or the U.S. -- and has provoked the threat of military strikes from both America and Israel.

Ahead of the start of Friday's session, the diplomat said EU Foreign Affairs chief Catherine Ashton, speaking on behalf of Iran's six interlocutors, would urge the Iranian side in her opening address to recognize the need to discuss international concerns about Iran's nuclear program.

Ashton, he said, would renew a 2008 offer providing Iran technical and logistical support for peaceful nuclear activities as well as trade and other incentives in exchange for its willingness to focus on its atomic program.

One development to watch for, he said, would be readiness by Iranian chief negotiator Saeed Jalili to meet U.S. counterpart William Burns in a bilateral meeting. While the Iranians met several delegations at the Geneva talks, they refused a U.S. overture to sit down one-on-one in the Swiss city.

Meeting the Iranians are the five permanent U.N. Security Council members -- the U.S., Russia, China, Britain and France -- plus Germany.

As a subset of the talks, discussions could be held on reviving an offer to exchange some of Iran's enriched uranium for fuel rods for Tehran's research reactor, said the diplomat.

First made in late 2009, that offer was supported by the six powers as a way of reducing Iran's enriched stockpile, thereby potentially delaying its ability to manufacture a nuclear weapon. But it lapsed over Iranian conditions and later the realization by Tehran's interlocutors that it no longer made sense to discuss shipping out the original amount as Iran continued adding to its enriched uranium trove.

The diplomat said any agreement to explore reviving those talks should be seen only as a confidence-building measure and should not detract from the ultimate goal of curbing Iran's enrichment activities.

It could be conditioned on Iran stopping the manufacture of 20 percent enriched uranium. Separate from its main enrichment program which is churning out low-enriched uranium, Iran started enriching to 20 percent after the fuel exchange deal was stalled, saying it would use the material to manufacture its own fuel rods for the research reactor.

That heightened international concerns, because it takes much less time to turn 20-percent enriched material into use for weapons than low-enriched uranium.

The nuclear talks are being held in the Ciragan Palace, resplendent with marble fittings, balconies and chandeliers, along the Bosporus strait, which divides Istanbul between the Asian and European continents. Fire destroyed the former Ottoman palace in the early 20th century, but the building was restored two decades ago and part of the grounds were turned into a five-star hotel, where some delegates to the talks are staying.

Tehran is under four sets of U.N. Security Council sanctions for refusing to cease enrichment and other activities that could be used to make nuclear weapons.

The Associated Press contributed to this report.