The European Central Bank kept its ultra-easy policy stance unchanged on Thursday, promising to hold rates low for an extended period and even maintaining a pledge to provide more stimulus if needed.

Six weeks after agreeing to halve asset buys from January, the bank reiterated its commitment to continue bond purchases at least until the end of September, and to keep reinvesting cash from maturing debt until much later to support a rebound in growth and inflation.

Having faced five years of anaemic inflation, the ECB has deployed its entire policy arsenal, cutting rates into negative territory, giving banks cheap loans and hoovering up bonds, all in the hope of boosting growth and rekindling inflation.

Its work has paid off as the eurozone recovery is now well into its fifth year thanks to nine million new jobs, letting policymakers curb stimulus from next year and raising the prospect that the lavish bond buys it started in early 2015 could finally end.

But the economic run is stronger than most have expected.

Indeed, the eurozone purchasing managers' index rose to a seven-year high this month, data on Thursday showed, while Germany's Ifo institute unveiled surprisingly bullish forecast for the eurozone's biggest economy.

This rapid expansion is fuelling arguments among more conservative policymakers that the ECB is moving too slowly and should be more decisive in signalling the end quantitative easing to preserve its remaining firepower after the crisis tested its limits.

Shifting the ECB's message on stimulus may have been made easier by a benign market reaction to the Federal Reserve's third, well-telegraphed interest rate hike this year late on Wednesday.

This was a sign that investors were confident enough in the state of the global economy not to fear a continued, albeit very gradual, increase in the cost of borrowing dollars - the currency that underpins much of world trade.

Indeed, policy hawks in Frankfurt are likely to have pushed Draghi to keep preparing markets for an end to asset buys sometime next year, arguing for changes in the bank's message to set up a formal decision by mid-year on ending a €2.55 trillion bond purchase scheme.