UPDATE 1-ECB likely to base bond buy on collateral rules-sources

* ECB to probably buy same covered bonds used as collateral

* Central bank officials say could spend more down the track

* Exit strategy to be for whole program, not just bonds

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May 27 (Reuters) - The European Central Bank will probably buy the same covered bonds under its asset purchase program that it accepts as security in central bank liquidity operations, euro zone central bank sources said. The purchases are unlikely to be unwound directly by selling bonds back at a certain date, central bank officials said, and the extra injection of liquidity will instead be offset in a broad-based exit strategy further down the track.

"Everything that you can use as collateral as a covered bond ... you can imagine that that would come under the umbrella of the outright purchase program," a euro zone central bank official said.

Two other officials, who spoke to Reuters on condition of anonymity, said the ECB was open to spending more than 60 billion euros ($83.7 billion) depending on market developments after its initial intervention.

"There is no discussion now of purchasing more," a second senior official said, adding that the initial spend could fall short of 60 billion if markets showed little appetite to sell.