Spain’s cost of borrowing money continued its rise higher into unsustainable territory Tuesday, reflecting deepening market fears that the country may eventually need a bailout Europe might not be able to afford.


In an auction of short-term debt, the Spanish Treasury was force to pay a yield, or interest rate, of 5.074 percent. That compared with a yield of 2.985 percent, itself a lofty level, only a month ago. And it comes the day after Spain’s 10-year bonds reached their highest rate since the creation of the euro currency union.


“The market is very uncertain that the euro will survive and, if that is the case, you don’t touch this debt with a barge pole,” said Christel Aranda-Hassel, senior European economist at Credit Suisse in London. “You need to break this fear that the euro is on the verge of breakup.”


On Tuesday Spain sold €2.4 billion euros, or $3 billion, of 12-month bills at the 5.074 percent rate. It also sold 640 million euros of 18-month debt with a yield of 5.107, compared with 3.302 percent last month.


Madrid is also hoping to sell up to 2 billion euros in longer-term bonds on Thursday.


The yield on Spanish 10-year bonds, considered the benchmark for borrowing costs, was at 7 percent early Tuesday afternoon. On Monday, that yield rose as high as 7.2 percent on Monday, a record since the inception of the euro and a level seen by many — including the country’s economy minister, Luis de Guindos — as unsustainable in the long term.


“They can keep going like this for certainly a couple of months, but obviously you don’t want to be doing this forever,” Ms. Aranda-Hassel said. “The pressure is definitely on the euro area member states to figure out how they break the deadlock.”


The Spanish government in Madrid has some flexibility in the amount of debt it needs to sell now, as it has already covered about three-fifths of its borrowing needs for the full year through previous sales. Spain’s Treasury accelerated its scheduled bond sales in the first quarter in order to take advantage of long-term loans provided by the European Central Bank to banks that bought hefty amounts of domestic debt.


But the rise in borrowing costs has raised the possibility that the Spanish government could need a full rescue package of the type other troubled euro countries have sought — and that Spain has tried desperately to avoid.


“By issuing debt at such an interest rate, Spain is putting at stake a sovereign bailout in the coming months,” said Fernando Ballabriga, economics professor at the Esade business school in Barcelona.


Spain has moved back to center stage in the euro crisis after legislative elections in Greece over the weekend brought to power a center-right political party that favors remaining in the euro.


The Greek vote was welcomed in Madrid on Monday by Prime Minister Mariano Rajoy as “very good news for Greece, for the European Union, for the euro and also for Spain.”


But while the victory of the party, New Democracy, removed for the moment the immediate threat of dissolution of the currency zone, it does not address fundamental problems in Spain and elsewhere in the euro zone.


Spain’s request, earlier this month, for a 100 billion-euro bailout for its banks left financial markets unimpressed — partly because the move will add to the country’s debt burden.


Market confidence in Spain’s bank rescue deal has also been undermined by the fact that its exact terms are yet to be negotiated.


“By admitting that it is no longer capable of propping up its banks, the Rajoy government has sent a message to the markets that the sovereign is in need of external support too,” Nicholas Spiro, managing director of Spiro Sovereign Strategy, a London consulting firm that assesses sovereign debt risk, wrote in a note Monday.


“In the realm of investor perceptions, Spain has crossed the Rubicon from solvency to insolvency. The markets are treating Spain’s bank-focused bailout as a pregnancy: there’s no such thing as a partial one.”


International pressure is mounting on European leaders to take decisive action over the euro debt crisis when they meet for a summit in Brussels at the end of next week.