(Reuters) - Euro zone banks that failed or barely passed this year's health checks will have to demonstrate they can make sustainable profits and may need to sell off loss-making units, the European Central Bank's top supervisor said.

Daniele Nouy told Reuters on Friday that simply finding more capital to plug shortfalls uncovered by the stress test may not be enough.

"Partly because of the financial situation in Europe and partly because of the structure of the banking systems in Europe ... the sustainable profitability is probably the main challenge and the main risk for banks in the coming years," Nouy told Reuters. "They have to establish sustainable profitability."

She added: "There are a number of banks that went through the comprehensive assessment, but might not go through next time."

From this month, the ECB became the direct supervisor for the single currency area's top 120 banks like Societe Generale and Deutsche Bank with powers to force them to top up their capital buffers or make other changes to keep them safe and sound.

Nouy has already asked some of the banks who struggled in the stress test to resubmit their plans for reinforcing their capital buffers.

"When we review the capital plans of the banks with a shortfall now it's totally about business model, it's the most important dilemma," she said.

"A lot of bankers are not in denial. They may not be rushing to take measures, but they know this is something they will have to address."

The ECB will launch a "supervisory campaign" next year that will scrutinize profitability of lenders, and home in on any concentration of risks or dangers from poor quality loans on their books, and risks from misconduct.

Banks will have to consider whether they should be holding on to loss-making branches or subsidiaries that are not core to the business.

"A lot of significant losses were taken in places that were of small importance vis-à-vis the rest of the group," Nouy said, adding they should consider selling off such units.

"We are not considering that one category of business model is better than another," she said.

She said it was unclear whether the next stress test of major lenders in the wider European Union will take place in 2015 as the "post mortem" of this year's health check was still underway.