UPDATE 1-S-Oil inks 20-yr deal with Aramco to secure oil supplies

* S-Oil says "highly unusual" deal to ensure stable supply

* Deal comes as Seoul faces possible Iran oil disruption

* S.Korea's Lee visiting Gulf to secure stable crude supply (Adds details and quotes)

SEOUL, Feb 9 (Reuters) - S-Oil said on Thursday it signed a 20-year contract to buy crude from Saudi Arabia as South Korea seeks to secure supplies and reduce dependence on Iran under pressure from the United States to halt doing business with the Islamic Republic.

President Lee Myung-bak is touring Gulf producers this week to tap alternative sources to feed Asia's fourth-largest economy's energy demand. The pressure to stop dealing with Iran increased after the United States imposed fresh sanctions targeting financial institutions and trade.

Saudi Aramco owns the largest stake in S-Oil, with 35 percent, and the South Korean refiner does not buy any oil from Iran. But the agreement is an assurance to the nation almost entirely reliant on imported energy of steady supplies for its third-largest refinery.

The long-term supply contract is "highly unusual" in a market where one-year supply contracts are the norm, S-Oil said in a statement.

S-Oil's refining capacity is 669,000 barrels per day (bpd) and Saudi Aramco currently supplies almost all of the crude it processes into fuels.

The contract will cover all the crude S-Oil needs for 20 years, a spokesman at the refiner said. Saudi Aramco's total annual supply volume to S-OIL is approximately 230 million barrels, the statement said. The statement and the spokesman didn't specify what terms the crude will be sold under.

Saudi Arabia, the world's top oil exporter would "actively consider" measures, including offering additional crude supplies, if Seoul asked for them, South Korean presidential office said in a statement on Wednesday.

South Korea has four refiners -- SK Energy, GS Caltex, S-Oil and Hyundai Oilbank. Only SK Energy and Hyundai Oilbank import Iranian crude. (Reporting by Cho Mee-young; Editing by Jonathan Hopfner, Manash Goswami)