An Iranian proposal to close down a third of its centrifuges and relinquish most of its low-enriched uranium has led to progress in talks with the six world powers in Geneva, according to Western diplomats. However, many issues remain unresolved and the chances of reaching an agreement on Iran’s nuclear program by March 30 are low, they add.

With Prime Minister Benjamin Netanyahu scheduled to leave on Sunday morning for Washington, ahead of his controversial speech to Congress on Tuesday, senior Israeli officials appraised of developments in the talks expressed concerns to representatives of the world powers.

The two previous rounds of talks between Iran and the six powers (the United States, Russia, China, Britain, France and Germany), in Munich and Geneva, focused on finding a formula that would keep Iran a year from obtaining enough high-level enriched uranium to manufacture one bomb.

The two major components of the formula are the number of centrifuges Iran will be allowed to keep and the size of the stockpile of low-enriched uranium that it could retain.

The Western diplomats said that in the Munich talks early last month, the powers made a new offer to the Iranians, in which for the first six years of the agreement Iran could keep operating some 5,000 out of 9,400 old-model centrifuges, while 4,400 would go off-line in such a way that it would take a long time to reconnect them.

In the four following years, according to the Munich proposal, Iran would be able to increase its centrifuges to 7,800, and over the five ensuing years to 9,400, the number it currently operates. The proposal also called for most of the low-enriched uranium Iran currently has (about 6 tons) to be sent to Russia, where it would be converted into nuclear fuel for the reactor at Bushehr, Iran, leaving a symbolic 300-350 kilograms in Iran.

The diplomats said that, according to the proposal, the centrifuges Iran would keep would be reconfigured so they could only enrich a smaller amount of uranium. The deal also included an extensive reconfiguration plan of the enrichment facility at Natanz, proposed by the United States.

At the end of the Munich talks, the Iranians said they would give their answer at the following round of talks, which took place in Geneva last week. Iranian Foreign Minister Mohammad Javad Zarif came to those talks, together with the head of Iran’s atomic energy agency, Ali Akbar Salehi, a close associate of Iran’s supreme leader, Ayatollah Ali Khamenei.

The Western diplomats noted Salehi had said Iranian nuclear scientists had examined the proposal and decided it was too complex to implement, especially because of the reconfiguring plan for Natanz.

The Iranians surprised the representatives of the six powers when they presented their own alternative proposal, the diplomats said.

That proposal included, for the first time, concessions regarding their stockpile of enriched uranium, as well as a bid to cut the number of old-generation centrifuges by one third.

The Iranians proposed they keep 6,000 centrifuges out of the 9,400 for the first 10 years of the agreement, and keep 500 kilograms of their low-enriched uranium, or, alternatively, to operate 6,500 centrifuges and only retain 300 kilograms of their low-enriched uranium, the diplomats said. After 10 years, with only five years left on the agreement, the Iranian proposal would gradually increase the number of centrifuges to the number they have today, the diplomats added.

However, among the unresolved issues is the Iranian demand that all sanctions be lifted immediately upon signing the agreement, whereas the United States and the other powers want the sanctions lifted gradually if Iran is seen to be meeting its obligations.

Another stumbling block is Iran’s continuing refusal of the world powers’ demand to fully open all aspects of its military nuclear program to inspection by the International Atomic Energy Commission.

The diplomats say that, considering the issues still in dispute, it is difficult to imagine the parties coming to an agreement by March 30. If such an agreement does emerge, because of an Iranian and American need to show progress, it will be a general document of principles only and will not include details on the outstanding bones of contention.

Another round of talks is scheduled for this Thursday in Switzerland. These meetings are to be preceded tomorrow by a meeting in Switzerland between U.S. Secretary of State John Kerry and his counterpart, Zarif. Salehi will also be present at that meeting.

At a press conference over the weekend, senior U.S. officials said “significant progress” had been made in the talks, but there were still many obstacles and they did not expect an agreement to emerge from this week’s talks.

Israel has been updated on the Iranian proposal in Geneva, which has given rise to greater concern in Jerusalem. Since the first round of talks in Geneva, National Security Adviser Yossi Cohen and Strategic and Intelligence Affairs Minister Yuval Steinitz have expressed their concerns to representatives of the powers.

A senior Israeli official said the two Israelis warned that the proposal leaves too many centrifuges in Iranian hands, does not include total dismantling of the off-line centrifuges, and does not address issues such as Iranian research and development of advanced centrifuges during the period of the agreement. Steinitz and Cohen also said they did not believe Iran would send its low-enriched uranium abroad.

Netanyahu will address Congress on Tuesday evening (Israel time). He is also due to speak to the pro-Israel lobby, AIPAC, tomorrow. In both speeches, Netanyahu will argue that the agreement with Tehran is “bad and dangerous,” and call for additional sanctions on Iran.

“I respect President Barack Obama and believe in the power of the ties between Israel and the United States, and their power to overcome differences,” Netanyahu said on Saturday, during a visit to the Western Wall. “As prime minister, it is my duty to ensure the security of Israel, and therefore we strongly oppose the agreement developing with Iran. We must explain the dangers stemming from this agreement to Israel, the region and the whole world,” Netanyahu added.

The White House plans to deal with Netanyahu’s speech by presenting counterarguments in a series of public appearances with Israeli officials. White House National Security Adviser Susan Rice is expected to address the AIPAC conference, to try and persuade the audience that the emerging agreement is the best option. “The challenge is for those who oppose the agreement like Netanyahu. They must present an alternative that will produce better results,” a senior U.S. official said over the weekend.