Turkish officials say they believe a deal on Iran’s nuclear programme is close, as PM Recep Tayyip Erdogan flew to join talks in Tehran.
Mr Erdogan will try to persuade Iranian leaders to allow their nuclear material to be sent abroad for processing.
Brazil’s President Luiz Inacio Lula da Silva is also at the talks.
The West, worried that Iran is trying to build a bomb, has warned of stiffer sanctions if the mediation fails. Iran denies having a weapons programme.
The Turkish foreign ministry said a formal announcement on the new deal could be made on Monday.
Last-chance meeting
Last year, Western powers proposed that Iran transfer its stockpiles of low-enriched uranium to Russia and France, who would process it into a form usable in a research reactor before returning it.
The deal was an attempt to allow Iran the benefits of nuclear energy without the concern of it having weapons capabilities. But Tehran rejected the idea.
The current talks with Brazil and Turkey, two non-nuclear states on friendly terms with Tehran, are an attempt to resurrect that plan but reportedly with Turkey as the country where the uranium would be sent.
"We thought that we should also go there, in case the exchange takes place in Turkey," said Mr Erdogan.
"I guarantee that we will find the opportunity to overcome these problems, God willing."
Both Russia and the US say the talks represent Iran’s last chance to avoid harsher sanctions.
Mr Lula arrived earlier and held talks first with Iranian President Mahmoud Ahmadinejad and then with spiritual leader Ayatollah Ali Khamenei.
After the meetings, Mr Lula said the level of hope of reaching an agreement "has increased".
The BBC’s Iran correspondent Jon Leyne, reporting from London, says the country has given mixed messages about a fuel-swap deal.
He says officials have suggested they are still open to the idea, but have then imposed conditions that the West would not accept.
Iran has been mounting a big diplomatic effort to prevent new UN sanctions; the foreign minister has travelled to all 15 members of the security council.
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