The ECB needs to conduct policy in a way that ensures that the 17-country region sharing the currency is not ensnared again by crisis, Governing Council member Ewald Nowotny said at an economic conference.
"It is more important here to further the expansion and thus avoid falling into a second great crisis," he told reporters when asked about the risks of keeping interest rates low for a long time.
ECB Executive Board member Benoit Coeure told a joint news conference: "We will keep our monetary policy accommodative as long as needed - the governing council has been clear on that," but also noted it needed to keep an eye on the consequences.
Coeure cited the risk that "a protracted period of low interest rates would not create the right incentives for reform in the financial sector, in particular for the restructuring and cleaning up of the balance sheets of euro area banks."
He said the ECB was still evaluating potential policy steps. "In exceptionally difficult times we have the duty to consider all options with an open mind," he said.
The men appeared to differ on how the ECB should start supervision of major banks in the euro zone, a step supposed to begin by mid-2014 after a health check of big banks' assets.
Nowotny, who is also head of the Austrian central bank, said his personal view was that a staggered start might be advisable given the responsibility of overseeing around 130 banks with assets worth around 25 trillion euros ($33 trillion).
Effective banking supervision needed clarity on how to wind up ailing lenders and fiscal backstops that may be required for this, he said, and Europe lacked this clarity.
"Just from a technical perspective it might make sense to start with a smaller number," he said. This would allow the bank asset quality review to be handled in a less risky way and make it easier to line up a fiscal safety net for laggards.
Staggering the start of the single supervisory mechanism (SSM) could also mean phasing in the asset-quality review, he said, noting that covering cover banks' worldwide activities was "quite a challenge".
Coeure countered that the ECB could stick to the agreed timetable without having to sacrifice "seriousness".
"If at any point in the process we see a tradeoff between quality or trust in the new SSM and the timeline then quality will be more important," he said.
Coeure said the review would combine scrutiny by national supervisors, a peer review among national authorities "and intervention of outside experts, probably in a limited way, so as to enhance the scrutiny on the evaluation and the assessment of the assets".
(Reporting by Michael Shields and Georgina Prodhan; Editing by Ruth Pitchford)