Political uncertainty over a corruption scandal and revived concerns over the economic health of the euro zone forced the Spanish Treasury to pay more to borrow at a triple-bond auction today.

Demand was strong, however, and yields remained well away from crisis levels.

Spain sold €1.948 billion in a 2015 bond, with the yield rising to 2.823%, up from 2.476% at the last sale of that paper, in January.

Yields also rose on a 2018 bond and a 2029 bond that were sold at the auction.

In all, Spain sold €4.6 billion worth of the three bonds, slightly higher than the top end of its target range. Demand was strong, continuing the trend from January when Spain saw yields on shorter-term paper falling to ten-month lows.

It means that Spain had now sold over 18% of its full year medium and long-term funding target.

Analysts said the result of today's auction reflects the recent shift in sentiment towards Spain: a marked increase in yields after months of declines.

Yields have jumped back up to mid-December levels in Spain and elsewhere in Europe, as an economic downturn spreads around the entire euro zone.

Investors are also eyeing potential political instability in Spain due to a widening corruption scandal involving officials of the ruling People's Party.

Prime Minister Mariano Rajoy has denied any wrongdoing in a graft scandal. A former PP treasurer, Luis Barcenas, appeared on Wednesday for questioning by prosecutors who are looking into reports that he ran a slush fund that allegedly channeling business contributions to party leaders. Barcenas has denied the accusations in the press.