While the bank's Governing Council left policy unchanged this time around, it was the clearest hint yet of more stimulus to come as new national lockdown measures make a double-dip recession increasingly likely for the euro zone economy.

"We agreed, all of us, that it was necessary to take action and therefore to recalibrate our instruments at our next Governing Council meeting," ECB President Christine Lagarde told a news conference.

Sources told Reuters some policymakers at Thursday's meeting favoured expanding the ECB's Pandemic Emergency Purchase Programme (PEPP) while others put greater emphasis on even more generous credit for banks under its Targeted Longer-Term Refinancing Operations (TLTRO), or on a mix of both.

One of the open questions related to the size and composition of PEPP, which has exhausted nearly half of its 1.35 trillion euro ($1.60 trillion) firepower, and how to make it targeted to the current juncture, one of the sources said.

Lagarde said ECB staff were already working on "all instruments".

"We stick with our forecast of a 500 billion euro expansion to the PEPP programme we've pencilled in since June, but we think more is coming in parallel," Frederik Ducrozet, a strategist at Pictet Wealth Management, said.

The euro fell sharply against the U.S. dollar after Lagarde spoke and was last trading down 0.7% at $1.166. Government bond yields fell.

Lagarde repeated several times that risks to the region's recovery were "clearly tilted to the downside" and heavily dependent on the success of efforts to contain a new wave of infections threatening to overwhelm Europe this winter.

The bloc's biggest economies, Germany and France, announced new lockdowns overnight. Others among the 19 countries that use the euro are also shuttering large parts of their services sectors, a blow to the fledgling recovery.

Buying roughly 100 billion euros ($118 billion) of debt a month, the ECB has already pushed borrowing costs to record lows, and spreads between the borrowing costs of euro zone members are back to pre-pandemic levels.

But there are clear limits to its powers and Lagarde stressed there should be no delay to the European Union's 750 billion euro recovery fund, affirming her view that monetary policy should be complemented by massive government support.

"An ambitious coordinated fiscal stance remains critical," Lagarde said, adding that she would not be surprised if individual governments offered more fiscal support given the worsening conditions.

DEEPENING GLOOM

The ECB's problem is that fresh COVID-19 restrictions are challenging its view that the euro zone economy will grow back to its pre-crisis level by the end of 2022.

Inflation expectations, the ECB's main worry, are also declining. While the threat of deflation is not yet back on the agenda, inflation may fall short of the ECB's target of nearly 2% for many more years to come.

"The December staff projections will likely show that a return towards the ECB's inflation objective could be further delayed," said Elga Bartsch, head of macro research at the BlackRock Investment Institute.

New COVID measures in France mean people must mostly stay at home, going to work only if their employer deems it impossible for them to do the job remotely. Schools will stay open.

Germany, whose economy was already losing steam, will shut bars, restaurants and theatres in November, though schools will stay open and shops will be allowed to operate with strict limits on access.

Spain, one of Europe's worst COVID-19 hotspots, where the government is planning to announce a six-month state of emergency, may already be back in recession, while Italy has also unveiled new restrictions.

Lagarde said that how the virus is managed between now and the end of the year would determine what side of zero the fourth quarter's growth number falls on.

"The ECB was there for the first wave, the ECB will be here for the second wave," she said.

(Writing by Mark John; Editing by Catherine Evans and Jon Boyle)

By Francesco Canepa and Balazs Koranyi