The United States and its allies are pressing for an end to Iran's high-level uranium enrichment and the closure of a facility built deep under a mountain as talks on Tehran's nuclear standoff with the West resume this week.

Iranian media said the talks, which collapsed more than a year ago, would be held in Istanbul on Saturday, Reuters reported.

A return to the table, as the Western allies tighten sanctions over what they say is Tehran's program to develop nuclear weapons, had been in doubt after Iran and the P5+1 countries -- the United States, Britain, France, Russia, China and Germany -- released conflicting statements about the venue.

Tehran had earlier voiced concerns about holding talks in Turkey, whose opposition to Iranian ally President Bashar al-Assad in Syria has angered the Islamic Republic.

After weeks of debates, Iran and the six world powers agreed to attend a first meeting in Istanbul, the semi-official Fars news agency reported, citing unnamed sources. State-run English language Press TV carried the same report.

Fars also said the sides had agreed to a second round of talks in Baghdad if there was progress in Turkey. There was no immediate comment on the venue from the world powers.

Turkish Prime Minister Tayyip Erdogan said the question of where the talks would be held was not yet confirmed.

A senior U.S. official said getting Iran to suspend high-level uranium enrichment and close a nuclear facility built deep under a mountain near the holy city of Qom are near-term priorities for the United States and its allies.

The New York Times said the United States and other Western nations planned to demand Iran immediately close and ultimately dismantle the Fordow facility and also would call for a halt in the production of 20-percent enriched uranium.

The U.S. official told Reuters 20 percent and closing Fordow are near-term priorities for the Obama administration and its international partners in dealing with Iran.

An Iranian official rejected any demand that Iran suspend enrichment of uranium to 20 percent and close the Fordow site.

We see no justification for such a request from the P5+1, the head of Iran's atomic energy organization, Fereydoun Abbasi-Davani, told the Iranian student news agency.

Iran says its nuclear program is for power generation and producing isotopes for medical purposes, but the U.N. Security Council has demanded a full suspension of enrichment, both to the 20 percent and the 3.5 percent level.

Iran has enough 3.5 and 20 percent-enriched uranium for around four bombs if the material is refined further to about 90-percent purity, Western experts say.

Earlier Sunday, Israel, which has also demanded an end to all enrichment and has threatened attacks on its arch-foe's nuclear facilities if diplomacy fails, signaled it would accept, as a first priority, the Western powers focusing on stopping 20-percent enrichment.

We told our American friends as well as the Europeans that we would have expected the pressure for successful negotiations to be clear - namely that the P5+1 will demand clearly, no more enrichment to 20 percent [and] all the already enriched 20 percent material out of the country, Israeli Defense Minister Ehud Barak told CNN on Sunday. But if the P5+1 will settle for a much lower threshold, like just stop enriching to 20 percent, it means that basically the Iranians, at a very cheap cost, bought their way into continuing their military program. Slightly slower but without sanctions. That will be a total change of direction for the worse.

The Israeli defense minister added that while oil and banking sanctions have clearly had an impact, causing inflation in Iran to nearly double to 21.5 percent in urban areas, he doubted they will be enough to force Tehran's hand.

We hope for the better but I don't believe that this amount of sanctions and pressure will bring the Iranian leadership to the conclusion that they have to stop their nuclear military programme, he said.

In Israel, Prime Minister Benjamin Netanyahu said: We saw in the past Iran has used the talks to try and delay and mislead the West, so the demands must be clear.

One, to end all enrichment of uranium, both 20 percent and 3 percent, secondly to take the enriched material out of Iran -- it is possible to provide them with alternative material for peaceful purposes -- and thirdly the illegal installation in Qom must be dismantled.

U.S. President Barack Obama has sent out several messages to Tehran in recent days warning that this is perhaps the last chance to negotiate an end to the crisis, the Guardian reports. The most recent was passed via the Turkish prime minister, Recep Tayyip Erdogan, at a meeting in South Korea on nuclear non-proliferation.

Secretary of State Hillary Clinton has also pressed the message that time is running out for negotiations. She said last week that the U.S. was taking part in the talks in Turkey because they may be a last chance to demonstrate a way forward that can satisfy the international community's concerns and have Iran come forward and accept limitations on what they are able to do.

Clinton warned that she did not want the Iranians to go into it with the attitude of that we can just keep it open and never have to come to any outcome.

While Iran says it has a right to peaceful nuclear technology, it has at times appeared more flexible regarding 20 percent enrichment, which it began in early 2010, and some experts say that initially getting Iran to stop this higher-grade work could open a way to ease the deadlock.

It is not clear what Iran would expect in return, but an easing of some of the sanctions, which include an oil embargo by the European Union, might be among them.

There was scant encouragement from Iran to signal it was ready for concessions.

In an address on Iran's annual day to celebrate its nuclear achievements, President Mahmoud Ahmadinejad warned Western powers that Iran would continue to pursue its swift scientific progress.

The nuclear industry is like a locomotive that can carry other industries along with it. It is like the space industry that has raised tens of sub-industries under it and it is clear that we must continue on this path, the Iranian state news agency quoted him as saying.

You are blind if you think you can block scientific growth in Iran by martyring Iranian scientists, he added, referring to the killing of four Iranian scientists since 2010. Don't think you can stop this roaring river, and know that if you assassinate one scientist, hundreds and thousands will take his place.

Last week, a senior Revolutionary Guard commander, Massoud Jazayeri, was quoted as saying that if any country attacked Iran's nuclear program, Tehran would hit back at the U.S.

In the face of any attack, we will have a crushing response. In that case, we will not only act in the boundaries of the Middle East and the Persian Gulf - no place in America will be safe from our attacks, he said.

America, the Zionists and reactionary Arabs should pay attention that we will seriously confront them wherever the Islamic Republic's interests are threatened.