The European Central Bank will keep rates at a record low for as long as needed to raise inflation, and its interest rate guidance should be seen as "open-ended", policymakers concluded in June, according to minutes of their meeting published on Thursday.

The ECB guided markets for steady rates "through the summer" of 2019 at its meeting last month, when it also announced that it would shut a €2.6 trillion bond-buying programme in December, ending its unprecedented stimulus scheme.

But policymakers at the meeting felt a need to stress that rates would move only if inflation continued to rise back towards the bank's target of almost 2 per cent, the minutes showed.

"It was felt that the open-ended character of the state-contingent component (of the guidance) should be emphasized, with policy rates expected to remain at their present levels for as long as necessary to ensure... a sustained adjustment in the path of inflation," the ECB said.

Still, even with such a caveat, policymakers argued that progress in raising inflation has been substantial and they were confident that price growth would continue to move towards the bank's target.

With inflation on the rise and growth appearing to level off, rate-setters decided on June 14 to cut bond buys from October, then shut the scheme in December, satisfied that protracted stimulus is boosting inflation, even if slower than expected.

But they also pushed out expectations for the bank's first interest rate hike, using a relatively vague wording that could include any of the four policy meetings in the second half of the year.

Markets are currently betting on the first move in October but expectations have swung sharply between July and December in the past few weeks, suggesting heightened uncertainly.

While policymakers said they "expected" to end asset buys in December, they also said that given prevailing uncertainty, it was prudent to keep the end of bond buys conditional on incoming data.

"It was widely cautioned that there were signs that the slowdown identified in the first quarter was likely to extend into the second quarter in a number of countries and would imply downside risks regarding the near term," the ECB added.

The risk from protectionist trade policies could quickly impact trade and growth but also carried the risk of market volatility.

"In this context, risks associated with heightened volatility in global financial markets were also highlighted more broadly," the minutes said.

In the long process of unwinding stimulus, the biggest complication is likely to be a murky economic outlook, muddied by a developing trade war with the United States, a populist challenge from Italy's new government and softening export demand.

Barring a crisis, the ECB is unlikely to walk back its plan to end the bond buys but rising global challenges increase the risk of even more protracted normalisation.