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SINGAPORE, May 9 (Reuters) - State-run Indian Oil Corp (IOC) bought 4 million barrels of West African and a rare 600,000-barrel cargo of Libyan Mellitah crude in its tender for July sweet grades, traders said on Friday.

IOC bought 2 million barrels of Nigerian Escravos crude at a premium of $4.20-4.30 a barrel to dated Brent, possibly the highest on record, traders added. The deals had yet to be confirmed.

The high premium comes amid continued unrest in Nigeria, expected by some traders to prompt IOC to turn to other grades.

But other traders said a switch to other grades was not easy. "There are problems of a rigid structure," a Singapore-based trader said.

IOC bought Escravos from European trader Vitol, traders said.

The grade may also have commanded a record premium due to its high middle-distillate yield at a time when the gas oil crack has surged again above $30.00 a barrel due to tightening global supplies.

The other 2 million barrels of West African crude comprised 1 million barrels each of Angolan Nemba and Girassol grades, both purchased from state-owned Sonangol, traders said.

A trader said IOC paid a discount of $1.90-1.95 a barrel to dated Brent for the Angolan grades but it was not yet clear if the price was the average level price of the two grades, or only one.

European major BP sold the cargo of Mellitah crude at a premium of around $2.45 a barrel to dated, which would be 45 cents higher than the last released official selling price for the grade, at a $2.00 premium for May.

IOC tenders several times a month to buy light sweet crude and once a month for heavy grades.

It owns about 10 refineries spread across India for a total capacity of 1.204 million barrels per day (bpd). (Reporting by Maryelle Demongeot; Editing by Ramthan Hussain)