Brazilian President Luiz Inacio Lula da Silva was in Iran on Sunday for a nuclear summit that major powers have said might prove to be Tehran's last chance to avoid new UN sanctions.
Ahead of his trip, Lula told reporters in Moscow he was "optimistic" and hoped to be able to persuade Iranian President Mahmoud Ahmadinejad to reach an agreement with the West.
"I must now use everything I have learned over my long political career to convince my friend Ahmadinejad to come to an agreement with the international community," he said.
Lula, who heads a 300-strong delegation, was officially welcomed by Ahmadinejad Sunday morning and the two leaders went into talks, local media said. A number of trade agreements will be signed later in the day.
Lula is also expected to meet Iran's supreme leader Ali Khamenei, who has the final say on all key policy issues.
But the United States and Russia have already said the chances of success are weak, while Turkey appears to have given up any hope of its neighbour avoiding sanctions over its controversial nuclear drive.
Brazil and Turkey, non-permanent members of the UN Security Council, have so far resisted US-led efforts to push through a fourth set of sanctions against Iran over its failure to heed repeated ultimatums to stop enrichment activity.
Iran has rejected a UN proposal to enrich abroad the uranium it says it needs for a nuclear research reactor. The West fears Iran wants highly enriched uranium to make an atomic bomb, a charge Tehran vehemently denies.
"We have received many proposals and we are considering them," Iran's atomic chief, Ali Akbar Salehi, was quoted as saying on Saturday in local media.
"There is a willingness on both sides to resolve the problem and things are moving positively," he added without elaborating.
But Foreign Minister Manouchehr Mottaki indicated that Iran was still not ready to budge from its dogged position.
"We hope that the parties (involved in the talks with Iran) will bend to the realities and choose the right path," Mottaki was quoted as saying in reports on Saturday.
Russian President Dmitry Medvedev has said that the Brazilian president's talks with Iran "may be the last chance before the adoption of appropriate decisions within the framework of the Security Council."
That came after a senior US State Department official told reporters on condition of anonymity that the Lula visit was being seen as "perhaps the last big shot at engagement."
Lula has in the past defended Iran's nuclear activities, saying Tehran has the right to atomic energy, and has repeatedly said sanctions would be counter-productive and ineffective.
The White House said President Barack Obama and Medvedev had discussed Iran by telephone this week and agreed to step up negotiations on a new sanctions package.
The two leaders noted the "good progress" and "agreed to instruct their negotiators to intensify their efforts to reach conclusion as soon as possible," said the White House.
Russia's agreement would leave China as the last remaining holdout among the five veto-wielding permanent members of the Security Council, bringing the prospect of sanctions much nearer.