Iran appears to have resumed converting small amounts of its higher-grade enriched uranium into reactor fuel, diplomats say, a process which if expanded could buy time for negotiations between Washington and Tehran on its disputed nuclear programme.
The possibility of Iran converting enriched uranium into fuel - slowing a growth in stockpiles of material that could be used to make weapons - is one of the few ways in which the nuclear dispute could avoid hitting a crisis by the summer.
Tehran could otherwise have amassed sufficient stock by June to hit a "red line" set by Israel after which it has indicated it could attack to prevent Iran acquiring enough fissile material for a nuclear weapon. Yet few expect progress in talks until after the Iranian presidential election in June - a formula for a potentially explosive clash of timetables.
Diplomats accredited to the International Atomic Energy Agency (IAEA) in Vienna told Reuters that Iran had apparently resumed converting into fuel small amounts of higher-grade enriched uranium - thereby reducing the amount potentially available for nuclear weapons - though they had few details and one told Reuters that "very, very little had been done" so far.
A fuller picture is unlikely until a new IAEA report on Iran's nuclear activity, due by late February. But the question is crucial in determining the size of its stockpiles and how close these are to Israel's red line. "We will all be doing the mathematics soon," said one diplomat.
In September, Israeli Prime Minister Benjamin Netanyahu said Israel would not let Iran acquire enough material for a bomb; enriching uranium raises the less than one percent of fissile isotope U-235 found in mined metal to higher concentrations: about 4 percent for reactor fuel, up to 90 percent for a bomb.
While scientists differ about how much uranium is needed to have the ability quickly to make a bomb, analysts say the Israeli figure is believed to be 240 kg of uranium enriched to 20 percent; at that concentration, the material is nine tenths of the way to the weapons-grade of about 90 percent, since most of the unwanted isotopes have been separated out by then.
"Israeli officials, in private, widely use the 240kg figure," said Shashank Joshi, a Research Fellow at Britain's Royal United Services Institute (RUSI). "The figure is so specific and so widely used that they must understand the implications of drawing this red line: that Iran is free to produce anything up to that amount, but that producing any more would force Israel to choose between humiliation or war."
Iran averted a potential crisis last year by converting around 100 kg of its 20-percent enriched uranium into fuel - prompting some analysts to believe it was deliberately keeping below the threshold for potential weapons-grade material set by Israel, while still advancing its nuclear technology. It is not believed to have enriched uranium beyond 20-percent.
Iran, a signatory to the Non-Proliferation Treaty, denies seeking nuclear weapons, saying its aim is electric power and some higher-grade enriched uranium for medical purposes. It says non-signatory Israel, assumed to have nuclear arms, is a threat.
Last year's fuel conversion only slowed Iran's accumulation of 20 percent enriched uranium and was stopped. As it continues to produce fresh supplies - diplomats believe it is adding 14 to 15 kg a month - stockpiles are rising quickly and they calculate Iran will hit the Israeli red line by May or June, unless it again expands fuel conversions or slows its rate of enrichment.
It is here that the complex calculations of nuclear experts and international diplomacy collide.
The Iranian nuclear programme is controlled by Supreme Leader Ayatollah Ali Khamenei - who last week publicly rebuffed US overtures for direct talks. While he is not facing voters himself, he is seen as unlikely to want to make any concessions until he has a firmer grip on the warring factions vying for power beneath him after the presidential election in June.
"I think, until we get a clearer sense of how that plays out, that the Iranians are going to be basically in a holding pattern," said Shannon Kile, a nuclear proliferation expert at the Stockholm International Peace Research Institute (SIPRI).
President Mahmoud Ahmadinejad said on Sunday Iran would not negotiate under pressure but would talk if others stopped "pointing the gun". At odds with Khamenei, Ahmadinejad will step down in June but appears to manoeuvring to maintain influence.
At the same time, there appears to be a growing recognition among world powers that using economic sanctions to force Tehran to curb its nuclear programme are unlikely to succeed without a broader political dialogue between the United States and Iran to ease acrimony dating back to the 1979 Iranian revolution.
US Vice President Joe Biden repeated an offer for direct talks at a conference in Munich early this month.
Negotiations with Tehran are currently run jointly by the five permanent members of the Security Council plus Germany - known as the P5+1. These are expected to make at best limited progress in a meeting with Iran due in Kazakhstan on February 26.
"It has been obvious for years that Iran would only move on this issue in the context of a direct dialogue with the US," said one former senior diplomat who has negotiated with Iran.
"Before that, it will continue to be a managed exercise in futility on the part of Iran waiting for this to happen, while mastering the technology in the process."
Taking the two together, the shifting diplomatic approach and advancing Iranian engineering, there would be a short window of time after June for any US-Iran talks to produce results.
After that, the progress of Iran's technology could hit new Western red lines, including reaching a perceived "breakout" capacity, where it could move from the ability to make a weapon to actually building a bomb fast enough to avoid detection.
How then, is Israel's red line to be postponed enough to allow time for diplomacy in the second half of the year?
Iran has shown no sign of slowing down the rate at which it enriches uranium to 20 percent in a plant at Fordow, diplomats say, though, perhaps significantly, it has not so far put into operation some new machinery - two cascades of inter-connected centrifuges which could have rapidly expanded that programme.
Based on data reported by the IAEA, the UN nuclear agency, Iran will hit Netanyahu's red line in May or June unless it converts more of its stockpiles into fuel, or slows enrichment.
An alternative scenario would be for Israel to blur the definition of its red line, given enough reassurance that its key ally the United States would be ready to stop Iran gaining nuclear weapons through diplomatic or military means.
Having lost seats in a parliamentary election last month at which many voters indicated they did not fully share his anxiety about Iran, Netanyahu may also be ready to bide his time.
With U.S. President Barack Obama due to visit Israel in March on a trip Netanyahu says will focus on Iran, Syria and the Palestinians, there are tentative signs Israel might give some space to the United States to pursue its diplomacy - though not necessarily on the issue of highly enriched uranium stockpiles.
"It is notable that, recently, there have been no new assassinations of Iranian scientists, no prominent covert action or explosions, and broad Israeli restraint on statements of military intent," said Joshi at RUSI in London, referring to a widely assumed covert campaign against Iran's nuclear programme.
"They are conceding US leadership on this issue - whether by choice or American design."
Former Israeli army intelligence chief Amos Yadlin said it was in Israel's interest that Washington or the P5+1 reach an agreement with Iran. Writing in a report by the Institute for National Security Studies in Tel Aviv, he said: "Such a solution is preferable to a strategy with two exclusive alternatives of 'an Iranian bomb' or 'the bombing of Iran'."