The European Central Bank failed to attract the 73.5 billion euros from banks on Tuesday needed to offset its seven-month run of euro zone government bond purchases, instead managing to draw just over 60 billion.
The pace of the ECB's government bond purchases picked up last week as the bank spent 1.121 billion euros reflecting the bank's ongoing attempt to calm euro zone debt markets.
The central bank takes seven-day deposits from commercial banks on a weekly basis to offset its spending, but the failure to fully sterilize the purchases is likely to reflect the fact banks are keeping hold of their funding around the traditionally tense year-end period.
It has happened before but I wouldn't make too much of a big deal out of it, said ING economist Martin Van Vliet.
The end of year is typically a quiet period and banks books are closed so it shouldn't be seen as a sign that tensions are returning to interbank markets.
The ECB paid a weighted average interest rate of 0.66 percent on funds deposited by banks compared with 0.42 percent last week.
A total of 41 banks offered up funds. (Click for full details) Last week 44 banks offered up 81.024 billion euros as the ECB sought to drain 72.5 billion euros.
The ECB can buy government and corporate bonds under its purchase programme but has not revealed how much it can spend or for how long it intends to buy them.
Last week's 1.121 billion euros worth of purchases was double the amount spent by the bank the previous week but still just a fraction of levels reached shortly after the programme was introduced in May.
Analysts believe the bank is concentrating its purchases almost exclusively on the government debt of euro zone troublespots Greece, Ireland and Portugal. They say it may have to increase its purchases in the new year when debt tensions are expected to return.
I think they (ECB) will have to ramp up their bond purchases next year when the pressures on periphery countries are likely to resume and the refinancing calendar gets busier, said ING's Van Vliet.
They are still in crisis mode, he added
(Reporting by Marc Jones)