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Mario Draghi, the president of the European Central Bank, arrives for Thursday's news conference. The bank revised its economic growth projections downward.Credit Ralph Orlowski/Reuters

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FRANKFURT — The European Central Bank opened the door to an increase in its economic stimulus program on Thursday as it issued a more pessimistic growth forecast. But the central bank will not act until it has more information about the effect of a slowdown in emerging markets and other risks, Mario Draghi, the bank’s president, said at a news conference.

“The Governing Council wanted to emphasize its willingness to act, its readiness to act and its ability to act,” Mr. Draghi said following a meeting of the council, the bank’s policy board.

For now, though, the central bank still needs to assess whether what is happening in the rest of the world is “worsening our medium-term outlook or is just a transitory thing,’’ Mr. Draghi said. “And then we’ll decide whether to do more or not.”

European stocks, which had already been up for the day, were further buoyed by Mr. Draghi’s remarks, illustrating his skill at reassuring investors without actually taking action.

The only concrete move by the European Central Bank at its meeting on Thursday was to raise self-imposed limits on how much it can buy of any single government’s or private lender’s bond issue. The change gives the central bank more leeway to increase the asset purchases that it began in March as a way of pumping money into the eurozone economy and forcing down interest rates.

In coming months, economists are likely to focus on whether, and how soon, the European Central Bank might see a need to step up that bond-buying stimulus program, known as quantitative easing. Currently the European Central Bank is buying government bonds and other assets at a rate of 60 billion euros, or about $68 billion, a month. It has said the purchases will continue at least until September 2016.

Mr. Draghi said that the central bank was prepared to adjust the “size, composition and duration” of that program if necessary.

The big challenges for the central bank include the threats to price stability in the eurozone and in export markets, as well as pressure on eurozone interest rates because of action under consideration by the Federal Reserve in the United States.

Mr. Draghi declined to comment on what effect Fed policy would have on the eurozone, except to say that if a rate rise is necessary to achieve “the objectives of the Federal Reserve’s monetary policy, this is a plus for the world.”

Staff economists at the European Central Bank scaled back their eurozone growth projections for this year and through 2017, citing “lower external demand” from emerging markets — a clear reference to the repercussions of the economic slowdown in China.

Growth in the 19 countries of the eurozone will be 1.4 percent this year, central bank economists said, compared to a forecast of 1.5 percent that they made in June. Inflation is now projected at 0.1 percent for the year, compared to an earlier forecast of 0.2 percent.

The central bank left its benchmark interest rate at 0.05 percent, where it has been for almost a year, and a change would have been a big surprise.

Unlike the Fed, which could begin raising interest rates as soon as this month, the European Central Bank is expected to keep its main rate at close to zero for some time. The annual rate of inflation in the eurozone is 0.2 percent, far below the official target of just under 2 percent. The European Central Bank does not expect inflation to be within striking distance of the target until 2017 at the earlierst.

With inflation so low and economic growth still weak, the central bank has no reason to raise interest rates. On the contrary, the European Central Bank’s bigger worry is about deflation, a broad, sustained fall in prices that is associated with unemployment and severe recession.

Mr. Draghi said, however, that he did not see a danger of deflation at the moment, even though there is a chance that the inflation rate could fall below zero. Falling oil prices are the main reason that inflation could decline further, he said.

A Greek journalist asked whether the bailout agreement that Athens reached this summer with its international creditors would allow the European Central Bank to start buying Greek government bonds as part of its stimulus program. Such purchases would increase the market demand for Greek bonds as well as their value.

Mr. Draghi said it was too soon to make that move, however. As Greece reaches various milestones in its bailout program in the coming months, he said, the European Central Bank would assess whether to begin buying Greek bonds.

Asked at the news conference to comment on the migration crisis in Europe, Mr. Draghi replied, “Any European should be horrified by the tragic loss of life happening on our doorstep.’’ But he added that the central bank had no policy role in that crisis, and that “the solution lies with elected political leaders.’’