International negotiations on Iran’s nuclear program have made dramatic strides towards a comprehensive agreement, according to diplomats at the marathon talks, now entering a critical last week before a major deadline, the Guardian reports.

The negotiators are due to gather once more in Lausanne on Wednesday, March 25, for a session that is due to last until Sunday, but could spill over until the following Tuesday, the deadline for producing a framework accord that lays out the main points of agreement. They would then have three more months to fill in the details for a comprehensive deal at the end of June.

The lead western negotiator, U.S. secretary of state John Kerry, met his counterparts from the UK, France, Germany and the EU at London’s Heathrow airport on Saturday and declared that “substantial progress had been made in key areas although there are still important issues on which no agreement has yet been possible”.

Diplomats in Lausanne confirmed that provisional agreement had been reached on a central issue that had defied compromise for years, Iran’s enrichment capacity. The mantra at the negotiations is that “nothing is agreed until everything is agreed” but the figures penciled in so far would allow Iran to run 6,000 of its current centrifuges for the lifetime of the deal, which would be ten to fifteen years. That would be a dramatic reduction compared to the current infrastructure of 10,000 operating centrifuges, and another 9,000 installed but non-operational.

The Iranian stockpile of low enriched uranium would also be radically reduced from thousands of kilograms to hundreds. The heavy-water reactor under construction at Arak would also be reconfigured so that it would be produce much less plutonium (the other possible path to a bomb).

The areas where no agreement has not been reached so far are the extent of research and development Iran would be allowed over the life of a deal, particularly development work on new model centrifuges, and the lifting of UN sanctions.

On the table in Lausanne, Iran was being offered a step-by-step lifting of UN sanctions, in return for “irreversible” steps it takes in dismantling nuclear infrastructure. At present, Tehran is rejecting that offer. On Saturday, the supreme leader, Ali Khamenei, demanded that sanctions should be lifted “immediately” and vowed that Iran would not submit to “bullying”.

The offer is also under fire from within the six nation negotiating group in talks with Iran. France signaled strenuously during the last two days of negotiations in Lausanne that it did not believe the bulk of negotiations should be removed until the International Atomic Energy Agency (IAEA) certifies that Iran has resolved all its questions about evidence of past development work on nuclear warhead design.