(Reuters) - Spain is negotiating with euro zone partners over conditions for aid to bring down its borrowing costs, though the country has not made a final decision to request a bailout, three sources with knowledge of the matter said on Thursday.

The favored option being discussed is that the existing European rescue fund, the EFSF, would purchase Spanish debt at primary auctions while the European Central Bank would intervene in the secondary market to lower yields.

No specific figure for aid has been discussed in the talks, which started several weeks ago, one of the sources told Reuters.

Other senior euro zone sources were more cautious, one saying nothing clear-cut had emerged on aid for Spain, while a fifth said no talks were going on at all.

Spain's prime minister's office declined to comment on the issue. A spokeswoman for the economy ministry said there was no change in the Spanish position, which is that it would wait until the next meeting of the governing council of the European Central Bank on September 6, hoping for details on how the ECB plans to intervene, before deciding on any move.

Earlier this month Prime Minister Mariano Rajoy inched closer to asking for an EU bailout for his country, but said he needed first to know what conditions would be attached and what form the rescue would take.

ECB President Mario Draghi has said the central bank could intervene to lower painfully high yields but only if the country concerned asked for similar help from the bloc's rescue fund first.

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