"Only a currency whose continued existence is not in doubt can be considered stable," Asmussen was quoted as saying by the Frankfurter Rundschau.

Draghi has called the euro "irreversible" and has said it could act to bring down high bond yields, which investors have driven up on speculation that the euro could break up. Investors would demand more interest to loan to countries if they fear their money is at risk of being converted into a different currency that is worth less.

German news magazine der Spiegel reported that the ECB was contemplating setting concrete yield caps above which it would intervene to drive borrowing rates down. The ECB replied in its statement, issued Monday, that "it is absolutely misleading to report on decisions which have not yet been taken."

The Bundesbank's objections to bond purchases are based on a view that they come too close to bailing out government finances, which the ECB is forbidden to do by the European Union treaty. Such purchases also could mean other countries could share in any losses on those bonds — a decision about the use of taxpayer money that belongs with governments, not the central bank.

To get the ECB to buy bonds, Spain would have to first apply for assistance to the eurozone's bailout fund, which could impose policy conditions such as reducing deficits or reforming the economy.

The ECB has not said how big the purchases would be or whether they would target a particular interest rate level. The bank's governing council next meets on Sept. 6.