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Nuclear diplomacy is set to gather pace in coming days, with Brazil鈥檚 president visiting Tehran over the weekend, hoping along with Turkey to find a last-minute compromise fuel swap deal between Iran and Western nations.

Yet even with recent signals from Iran toward rekindling a nuclear fuel plan that has languished for more than seven months, Turkey is tempering optimism that a diplomatic solution can be found in time. The US has been spearheading a push for a fourth set of UN Security Council sanctions on Iran to be imposed, possibly within weeks.

鈥淭here is nothing new going on鈥� from the Iranian side, said a senior Turkish Foreign Ministry official on Friday. 鈥淏ut let me tell you, on the other side, the track on sanctions is building up steam.鈥�

Turkey: C'mon, we need concrete steps

A discussed trip to Tehran by Turkey鈥檚 Prime Minister Recep Tayyip Erdogan to join his Brazilian counterpart Luiz Inacio Lula da Silva on Sunday is not likely, after a number of recent trips by foreign ministers between Tehran and Ankara. Turkey opposes sanctions, and currently holds a nonpermanent seat 鈥� like Brazil 鈥� on the 15-member Security Council.

鈥淚f [Erdogan] goes there 鈥� he wants something concrete to come out of this,鈥� the Turkish official told journalists on the condition he not be named. 鈥淏ut we need to make some progress, and we鈥檝e let that be known to the Iranians, that 鈥榗鈥檓on, we have to show something concrete now, otherwise it鈥檚 going to be very difficult for us to make some headway.鈥� 鈥�

A senior US State Department official told reporters in Washington that the Brazilian leader鈥檚 visit is 鈥減erhaps the last big shot at engagement鈥� before sanctions. US Secretary of State Hillary Clinton called her Turkish counterpart this week to say that the US would nevertheless keep pushing for sanctions.

Q&A: Will Iran nuclear sanctions work?

On Thursday, US President Barack Obama and Russian Dmitry Medvedev spoke by telephone; a White House statement afterward said the two 鈥渁greed to instruct their negotiators to intensify their efforts to reach conclusion as soon as possible.鈥�

Iran, US waffle on terms of nuclear swap deal

Iran has said it accepts in principle the original US-backed UN swap deal: to export the bulk of its homemade low-enriched uranium (LEU) to be further enriched in Russia and then turned in France into fuel rods designed for the small, decades-old reactor in Tehran that produces medical isotopes.

But Iran then rejected the deal, and has since come back with a counteroffer that would transfer its material in smaller batches to a third party on Iranian soil, and simultaneously receive the nuclear fuel it needs in return.

US and European officials have rejected that offer, because it does not achieve their primary purpose that the deal remove, in a stroke, Iran鈥檚 ability to enrich the leftover material to a much higher level 鈥� if it chose to do so 鈥� for a single nuclear weapon.

Also, American officials say that since the deal was first put forward in October, the 1,200 kg of LEU discussed then, which would have constituted 70 percent of Iran鈥檚 stock, no longer achieves the same aim. The new minimum figure would be closer to 2,000 kg.

鈥淚t鈥檚 been dragging on for months, and we don鈥檛 know 鈥� if one side is serious or not,鈥� says the senior official. 鈥淎t least we are serious. And we want them, the Iranians, to know that this is a very serious business.鈥�

鈥淚 think that鈥檚 an important distinction, and it鈥檚 making it more difficult for us to convince the Iranians that they have to do something,鈥� the official said. 鈥淭he fact that the American position has changed slightly, that 1,200 kilos won鈥檛 cut it anymore, and we need more, is probably another excuse for the Iranians to say, 鈥榊ou see? The Americans have changed the parameters.鈥� 鈥�

Reading the tea leaves in Tehran

Also complicating the job for would-be deal-makers Turkey and Brazil are the different power centers in Iran 鈥� and trying to read them, as they hold their meetings.

鈥淭here are so many different people involved in this,鈥� said the Turkish official. 鈥淲e don鈥檛 know if they are playing us 鈥� if the different power centers are real different power centers, or if they see eye-to-eye on this issue. So it鈥檚 difficult to gauge, actually, what is in the minds of the Iranians."

Iran denies wanting nuclear arms and says they are forbidden by Islam.

Speaking in New York two weeks ago, UN Secretary General Ban Ki-Moon said the 鈥渙nus鈥� was on Iran to accept the deal as a confidence-building measure.

Yet President Mahmoud Ahmadinejad, speaking shortly after the UN chief, said: 鈥淲e鈥檝e accepted that from the start鈥�. Therefore we have now thrown the ball in the court of those who should accept our proposal and embark on cooperation with us."

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