* Files point to bribery around trade with Iran
* Former ministers have denied wrongdoing
* Scandal is challenge to Erdogan ahead of elections
* Parliament to reconvene for extraordinary session
By Humeyra Pamuk and Nick Tattersall
ISTANBUL, March 14 (Reuters) - A Twitter account behind a string of leaks in a Turkish corruption scandal has posted what it presented as prosecution files accusing former government ministers of involvement with an Iranian businessman in a bribery and smuggling racket.
The posting late on Thursday is the latest blow to Prime Minister Tayyip Erdogan in a corruption scandal which has grown into one of the biggest challenges of his 11-year rule, and which he says is orchestrated by his political enemies to undermine him weeks ahead of important local elections.
Reuters could not verify the authenticity of the documents.
The Twitter account using the pseudonym @HARAMZADELER333 posted links to two documents, purportedly prepared by prosecutors and based on police files, from a long-running investigation that became public on Dec. 17 with a series of dawn raids.
Former interior minister Muammer Guler, former economy minister Zafer Caglayan and former environment minister Erdogan Bayraktar each saw a son detained in those raids. All three resigned just over a week later.
Former EU minister Egemen Bagis was replaced in a reshuffle.
The former ministers, all named in the documents, have denied any wrongdoing. They could not immediately be reached for further comment on Friday.
The first 299-page dossier contains what it says are wiretap transcripts, surveillance photographs and other documents it cites as evidence of a "criminal organisation" facilitating Iranian money transfers via gold smuggling and other forms of trade, including supposedly of food and medicine.
Iranian businessman Reza Zarrab is presented as the ringleader, with three of the former ministers, two of their sons, the former general manager of state-run lender Halkbank and other Turkish officials alleged to have been involved in facilitating his activities.
Zarrab's lawyer Seyda Yildirim said she could not comment because investigations were ongoing. Former Halkbank general manager Suleyman Aslan, also detained on Dec. 17 but released last month, could not be reached. Halkbank, which has repeatedly said its Iran dealings are entirely lawful, declined to comment.
U.S. officials have sought to prevent Turkish gold exports from providing a financial lifeline to Tehran, which has been largely frozen out of the global banking system by Western sanctions over its disputed nuclear programme.
Turkey has bought natural gas and oil from Iran through an indirect system whereby Iranian exporters received payment in Halkbank lira accounts and used that money to buy gold. The bulk of that gold is then shipped from Turkey to Dubai, where Iran could import it or sell it for foreign currency.
Turkish officials and foreign diplomats have repeatedly said that Turkey's energy and gold trade with Iran does not in itself breach international sanctions or contravene any laws.
But the purported prosecution dossier paints a picture of a network of bribery and smuggling around the trade itself.
In one example in the document, wiretapped phone calls between Zarrab and Caglayan's office are cited as showing how Caglayan intervened on Zarrab's request to prevent the seizure at Istanbul's Ataturk airport of a plane from Ghana carrying 1.5 tonnes of gold bullion without proper documentation.
Halkbank's Aslan is described in the prosecution document as actively involved in advising Zarrab on other ways of handling Iranian payments when the gold trade came under tighter scrutiny.
A commentary on parts of the evidence in the dossier said Guler had aided Zarrab by facilitating Turkish passports for him and his associates and by reassigning a police officer who grew suspicious about Zarrab's activities. Surveillance photographs in the dossier meanwhile purport to show Zarrab's associates delivering a shoebox of cash to Bagis.
Wiretap recordings purportedly of conversations between Zarrab and one of his associates are also cited as evidence that Guler wrote letters of reference to two Chinese banks vouching for Zarrab's activities, after his businesses in China started running into difficulties.
"Soon enough half of the cabinet is going to vouch for us," one of Zarrab's close associates is quoted as saying in the transcript of a telephone conversation.
Former environment minister Bayraktar is named in the second 32-page document, which details separate allegations of corruption in the approval of construction projects in Istanbul. Construction magnate Ali Agaoglu, also named in that dossier, told Reuters he had not read the file and declined to comment.
Police files on the four ex-ministers were sent to parliament in late February, where a summary would usually be read aloud to deputies. However, the assembly went into recess ahead of March 30 municipal elections period shortly afterwards and the files have been kept under lock and key ever since.
Parliament speaker Cemil Cicek, a senior member of Erdogan's AK Party, said parliament would reconvene for an extraordinary session on March 19 after the main opposition party demanded it be recalled to hear the allegations.
Erdogan has cast the graft investigation as a plot to smear him by Islamic cleric Fethullah Gulen, a U.S.-based former ally with influence in Turkey's police and judiciary.
Erdogan's advisers have said Gulen's network illegally tapped thousands of phones, including the prime minister's, over years to blackmail and concoct criminal cases as part of a campaign of covert influence over the state.
Gulen and his followers have denied orchestrating the corruption investigation or conspiring against the government.
The scandal has shown little sign so far of seriously weakening Erdogan ahead of the municipal elections. He remains fiercely popular in the conservative Anatolian heartlands for overseeing a decade of rising prosperity in Turkey, with most of the country's electoral map AK Party orange.