(Reuters) - Turkey said on Sunday Iran had agreed on a nuclear fuel swap deal which could help end Tehran's stand-off with the West over its atomic programme.

Full details of the agreement were not immediately released by Turkish and Brazilian officials mediating in Iran's dispute with leading world powers, who suspect Tehran of covertly developing a nuclear bomb.

Turkey's foreign ministry said a formal announcement might be made on Monday after any final revisions by the Brazilian and Iranian presidents and the Turkish prime minister.

"Yes, it has been reached after almost 18 hours of negotiations," Turkish Foreign Minister Ahmet Davutoglu told reporters in Tehran when asked if there would be an agreement.

Earlier, Turkish Prime Minister Tayyip Erdogan flew to Tehran to join Brazilian President Luiz Inacio Lula da Silva who has been negotiating with Iranian officials in what Western and Russian authorities have said is probably the last chance to avoid new U.N. sanctions against Iran.

A U.N.-backed deal offered Iran last October to ship 1,200 kg (2,646 lb) of its low enriched uranium -- enough for a single bomb if purified to a high enough level -- to Russia and France to make into fuel for a Tehran research reactor.

Iran later said it would only swap its LEU for higher grade material and only on its own soil, conditions other parties in the deal said were unacceptable. It denies seeking to build an atomic bomb.

"I am going to Iran because a clause will be added to the proposal which says the swap will take place in Turkey," Erdogan had said earlier.

"We will have the opportunity to start the process regarding the swap," he said. "I guarantee that we will find the opportunity to overcome these problems, god willing."

Lula also told reporters after holding talks with Iranians that "the level of hope (to reach an agreement) has increased."