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* First big pre-finance since lifting of sanctions
* Deal is mainly for refined products
* Iran clawing back market share from rivals
By Julia Payne and Rania El Gamal
LONDON/DUBAI, Jan 4 (Reuters) - The world's largest oiltrader, Vitol, has clinched a deal with the National Iranian OilCo. (NIOC) to loan it an equivalent of $1 billion in eurosguaranteed by future exports of refined products, four sourcesfamiliar with the matter said.
The pre-finance deal is the first such major contract signedbetween Iran and a trading house since sanctions were lifted inearly 2016. Vitol and NIOC declined to comment.
It highlights the speed of the oil industry recovery in Iranjust a year after lifting of sanctions, which is allowing Tehranto claw back oil market share from arch-rival Saudi Arabia.
It also re-establishes some old dealings with Western firmsas Tehran is benefiting not only from easing of EU sanctions butalso from reduced U.S. restrictions on its access to dollars,which Iran needs to reignite its economy.
Foreign companies still tread carefully for fear of breakinga myriad of complex laws, and oil majors such as Shell,BP and Eni have been slow to return as regularcrude lifters.
Executives who are U.S. citizens are often ring-fenced fromnegotiations with Iran, notably BP's CEO Bob Dudley and eventhose working for non-U.S. companies.
U.S. president-elect Donald Trump has also been outspokenabout reviewing the nuclear deal brokered under Barak Obama'sadministration, adding fresh uncertainty.
But privately held trading houses are more flexible and cannegotiate deals quicker than listed firms.
Traders have increasingly turned to pre-finance in recentyears to secure long-term access to large volumes of oil andproducts - the system of pre-finance by large traders includingVitol has for example kept the Iraqi region of Kurdistan afloatduring its war with Islamic State in the last two years.
The Vitol Iranian deal was signed in October and will comeinto effect this month, one of the sources who is based inTehran said.
"It is in euro...with the interest rate of around 8 percentin exchange for oil products," the source said, adding that someproducts could be supplied by the private sector rather thanNIOC.
Major crude producers in the Middle East, including Iran,remain reluctant to sell crude oil to traders as they prefer tocontrol pricing and destination themselves.
Traders have also been looking at restarting the Caspiancrude and product swaps with Iran but the process has been slowto pick up.
OPEC's third-largest oil producer, Iran, exports more than500,000 bpd of refined products, mainly fuel oil, petroleum gasand naphtha to Asian markets, according to OPEC. (Additional reporting by Dmitry Zhdannikov; editing by SusanThomas)
