Global powers raced against the clock Monday on the eve of a deadline to nail down the final pieces of a framework deal aimed at putting any Iranian nuclear bomb out of reach.

Adding to the drama, Russian Foreign Minister Sergei Lavrov left the crunch talks with Iran in Switzerland after a series of meetings, Russian media reported.

He will only return if there is a "realistic" chance of a deal, his spokeswoman said earlier.

Lavrov and his counterparts from the United States, China, Britain, France and Germany met with the Iranians in a lakeside Lausanne hotel on Monday for their first full session since missing a previous November deadline.

A Western diplomat said it was "yes or no" time, adding the talks remained blocked on three major issues -- the length of the accord, the lifting of UN sanctions and a mechanism to ensure both sides stick to the deal.

Global powers have set a midnight Tuesday deadline to agree the outlines of a deal that they will then try to finalise by June 30. Only then would Iran receive sanctions relief, diplomats said.

- Ample notice -

The global powers want Iran to scale back its nuclear programme to give the world ample notice of any dash to make the bomb and end a crisis that has threatened to escalate dangerously for 12 years.

The diplomatically isolated Islamic republic denies wanting atomic weapons and is calling for the lifting of sanctions that have strangled its lifeblood oil exports and its access to the global financial system.

The threat of new US sanctions, and domestic pressure on Iranian President Hassan Rouhani for his attempts at rapprochement with the West, all but rule out any further extension of the deadline.

"We had a very important meeting, and detailed discussions," Iran's lead negotiator Abbas Araghchi said, while insisting "we have solutions" for most of the remaining issues.

China's Foreign Minister Wang Yi said he was "cautiously optimistic" adding the differences between the parties were "narrowing."

Even before a deal is sewn up, opponents have railed against it, fearing it will not do enough to stop Iran getting the bomb.

These include US President Barack Obama's Republican opponents and Israel, widely believed to be the sole, if undeclared, nuclear-armed power in the Middle East.

Saudi Arabia -- leading an Arab coalition which on Monday carried out a fifth straight night of air strikes on Iran-backed rebels in Yemen -- is also uneasy about any thawing in US-Iran ties.

Israeli Prime Minister Benjamin Netanyahu, in his second broadside in two days, said Monday a deal would be tantamount to Tehran being "rewarded" for its "aggression" in Yemen.

- Jigsaw puzzle -

Western diplomats say some areas in a highly complex jigsaw puzzle are tentatively agreed. But they caution there is a long way to go.

One said Sunday that Iran had "more or less" agreed to slash the number of its centrifuge enrichment machines from 20,000 to 6,000 and to ship abroad most of its stockpile of low-enriched uranium.

This would make it a much more lengthy process to further purify these stocks to weapons-grade.

Iranian officials dismissed the numbers as "speculation", with Araghchi ruling out sending the stocks abroad, although he said "other options" were being examined.

This could include diluting low-enriched uranium or converting it to another form.

In addition to scaling down its nuclear programme, the powers want Iran's remaining facilities to be subject to an unprecedented level of inspections by the UN atomic watchdog.

Its underground facility at Fordo would also likely be barred from uranium enrichment, diplomats said, although it might be kept open for research purposes.

The United States, EU and others are only prepared to suspend their sanctions, not terminate them, and in a phased manner in order to ensure that Iran does not violate the deal.

The issue of UN Security Council sanctions is particularly tricky.

Araghchi said Sunday there must be a "precise framework" for lifting sanctions. The duration of any deal -- the United States wants at least 10 and possibly up to 15 years -- is also a point of contention.

With the world's fourth biggest oil and second biggest gas reserves, the energy industry is the cornerstone of Iran's economy, but it was hit hard by the American and European embargo imposed in 2012.

The sanctions sent the economy of the OPEC member state into recession but it is now reporting growth again.