The U.S. on Monday upped the ante in the Iran crisis and demanded that the Islamic Republic cease all uranium enrichment activity as a condition for a resumption of talks scheduled to begin this weekend in Istanbul.

In a move that seemed to indicate that the White House has adopted Israel's position on Iran, White House spokesman Jay Carney said on Monday "Our position is clear. Iran must live up to its international obligations, including the full suspension of uranium enrichment, as required by multiple U.N. Security Council resolutions. We are obviously aware of and following the situation at Fordo and Iran’s enrichment to 20 percent. Those are priorities for us and priorities for the international community. But our bottom line is the cessation of uranium enrichment and the verifiable decision by the regime in Tehran to forego pursuit of a nuclear weapon."

Carney added "It is important for Iran to understand that the window is closing and that these talks are an opportunity. The decision rests with Iran."

Iran's nuclear chief on Sunday signaled that Tehran's envoys may bring a compromise offer to the talks: Promising to eventually stop producing its most highly enriched uranium, while not totally abandoning its ability to make nuclear fuel.

The proposal outlined late Sunday seeks to directly address one of the main issues in the talks scheduled to begin Friday between Iran and the five permanent Security Council members plus Germany. The U.S. and others have raised serious concerns about Iran's production and stockpile of uranium enriched to 20 percent, which could be turned into weapons-grade level in a matter of months. Uranium has to be enriched to more than 90% to be used for a nuclear weapon.

Israel and other Western countries fear Tehran is seeking to develop an atomic weapon, a charge Iran denies.

Fereidoun Abbasi, Iran's nuclear chief, said Tehran could stop its production of 20% enriched uranium needed for a research reactor, but continue enriching uranium to lower levels for power generation. Abassi told Iranian state television late Sunday that this could take place once Iran has stockpiled enough of the 20% enriched uranium. Israel rejects this position as it would theoretically allow the Iranians to 'break out' and enrich to 90%, which is much easier and quicker to do with a stockpile of uranium enriched to 20%.

Last week, Iranian lawmaker Gholam Reza Mesbahi Moghadam claimed Tehran has the know-how and the capability to produce a nuclear weapon, but would never do so. Iran's supreme leader, Ayatollah Ali Khamenei, also has said that Iran does not seek nuclear arms and described them as being against the tenets of Islam.

Commenting on Iran's claims not to be interested in producing nuclear weapons, Carney said "Regardless of what the Iranians have said about what their intentions are, no one on the international stage has faith in those assurances."

U.S. State Department spokeswoman Victoria Nuland was also skeptical about Iran's assurances and said on Monday that "The Iranians themselves have said, at the level of the supreme leader, that they don't have any weapons intention. Well, if that is in fact the case, then it ought to be relatively straightforward for them to demonstrate that to the international community's satisfaction, and that's what we're talking about when we see them."

Iranian Foreign Minister Ali Akbar Salehi was quoted on the Iranian parliament's website Monday as saying he hopes for some progress in the talks. But he warned that Iran would not accept preconditions - an apparent reference to last year's impasse.

"We will honestly try to have the two sides conclude with a win-win situation in which Iran achieves its rights while removing concerns of five-plus-one group," Salehi said, using the name often used for the five permanent Security Council members and Germany. "But imposing any conditions before the talks would be meaningless."

On Monday, Finance Minister Yuval Steinitz told Italian Prime Minister Mario Monti - who is on an official visit to Israel - that he believes Iran's economy is on the verge of collapse, which is why sanctions against Iran must continue to be tightened.

Steinitz said Iran's nuclear program is not meant for peaceful purposes and that is how it must be viewed.

In what seems to be an internal conflict based on the upcoming talks, former Iranian president Ali Akbar Hashemi Rafsanjani, currently head of the Expediency Discernment Council, insisted that Iran must conduct direct negotiations with the U.S.

Rafsanjani denounced President Mahmoud Ahmadinejad’s positions concerning the U.S. and Saudi Arabia. "The current situation in which we are not talking to the U.S. and the U.S. is not talking to us, cannot continue. What is the difference between Europe, China and Russia, on one hand, and the U.S. on the other? If we talk to those countries, why don't we also talk to the U.S.?" Rafsanjani said.

Rafsanjani accused Ahmadinejad of causing the deterioration of Iran's relationship with Saudi Arabia, which he said was much better in the past.

Meanwhile, in a move that was sure to raise tensions further, the U.S. navy announced on Monday that it sent a second aircraft carrier, the USS Enterprise, to the Persian Gulf. The Enterprise, the world's first nuclear-powered carrier, joined the USS Abraham Lincoln and was said to be on a mission to support U.S. troops in Afghanistan and prevent pirate activity along the Somalian coastline.

A U.S. navy commander called the move "routine" and said it was not related to a specific threat. A White House spokesman denied two carriers would be present in the gulf and said the carrier already present there is to be replaced.