* Spain issues three 2014, 2015 bonds
* Yields fell compared with previous auctions
* Spain's borrowing costs have soared in euro zone debt crisis (Adds details, comment, background)
By Fiona Ortiz
MADRID, Oct 6 (Reuters) - Spain's borrowing costs fell on Thursday as it sold 4.5 billion euros of three 2014 and 2015 bonds, at the top of its target range, as European Central Bank support on the open market eases concerns of Madrid being sucked further into the euro zone debt crisis.
ECB buying of Spanish and Italian sovereign debt in the secondary market has buoyed prices for those countries' bonds, helping lower the yields they have to offer at debt auctions, as the two countries fight off doubts about their fiscal and economic health.
"The auction was successful probably because the European Central Bank's interventions have put a floor under bond prices and the yields are attractive compared with French or German bonds," Nicolas Lopez, research director at M&G brokerage in Madrid.
Italy and Spain, the euro zone's third- and fourth-largest economies, have been under the market microscope over the last two months amid concerns they would be too big to bail out if a worsening economic backdrop and contagion from default-threatened Greece made their financing costs unsustainable.
Last week Italy paid the highest yield on a 10-year bond since the introduction of the euro.
The average yield on Spain's April 2014 bond, in Thursday's auction, fell to 3.589 percent from 4.813 percent at the previous sale of the same paper. Yields on its October 2014 and April 2015 bonds also fell.
For a table of auction results, double click on
Concern over Spain's economic situation has eased somewhat as investors price in the likelihood the government will cut the budget deficit to 6 percent of gross domestic product as promised.
But uncertainty over debt levels in its 17 autonomous regions and potential writedowns for its banks from a burst property bubble continue to weigh, and the premium Spain pays for benchmark 10-year debt over the German equivalent remains near historic highs.
A three-notch downgrade of Italian debt by the ratings agency, while largely anticipated, highlighted the contagion risks from Greece, while talk of a plan to recapitalise European banks and possible International Monetary Fund backing for Spanish and Italian debt lifted the mood.
ECB purchases of Spanish and Italian have kept cash flowing in an otherwise stagnant market, some analysts said. Data on Monday showed the ECB bought 3.795 billion euros of bonds last week, slightly lower than the previous week.
Under the programme, the ECB and the 17 euro zone national central banks can buy government and corporate bonds from banks and other investors, but not directly from governments. (Additional reporting by Paul Day and Manuel Maria Ruiz; Editing by Toby Chopra)