The Federal Reserve closed a chapter on its aggressive, pandemic-driven stimulus on Wednesday, approving plans to begin tapering its bond-buying program this month, amid concerns that inflationary pressures may last longer than officials anticipated earlier this year.

Fed officials agreed to reduce their $120-billion-a-month asset-purchase program by $15 billion each in November and December, a pace that could see the purchases phased out completely by next June.

Fed Chairman Jerome Powell stated that officials had moved ahead of market expectations earlier this year, the potential end-date for the bond-buying program in case they decide they need to raise interest rates next year to cool down the economy if inflationary pressures broaden.

Mr. Powell downplayed the prospect of an impending rate hike, which boosted markets that had been rattled by other central banks’ recent policy tightening pivots.

On Wednesday, broad U.S. stock indexes set new highs. The S&P 500 gained about 0.7 percent. The yield on the benchmark 10-year US Treasury note increased to 1.577 percent on Wednesday, up from 1.546 percent the previous day. Bond yields and prices are inversely related.

It has also been purchasing at least $80 billion in Treasury securities and $40 billion in mortgage securities each month, initially to stabilize financial markets and later to keep longer-term interest rates low. Since March 2020, the Fed’s holdings of these securities have more than doubled to around $8 trillion.

The Fed stated that it would reduce asset purchases by $15 billion per month, but that it could accelerate or slow that pace “if warranted by changes in the economic outlook.” Officials do not want to raise interest rates until they have completed their bond purchases.

Mr. Powell declined to specify under what conditions the Fed might accelerate the reductions, or “taper,” of its asset purchases, causing some analysts to disagree after the meeting. Brisk demand for goods, disrupted supply chains, temporary shortages, and a rebound in travel have all contributed to 12-month inflation reaching its highest levels in decades. Core inflation, which excludes volatile food and energy prices, increased 3.6 percent year on year in September, according to the Fed’s preferred measure.

Diane Swonk, chief economist at accounting firm Grant Thornton, predicted that inflation would remain high enough for the Fed to feel pressured to accelerate the taper next year.

Mr. Dutta believes that as more people return to the labor force, wage growth will slow even as aggregate incomes rise, keeping inflation in check. “If Covid is the reason why people aren’t working,” he says, “then as Covid goes away, those things should look a little better.”

The Fed’s statement from April to September described high inflation as “largely reflecting transitory factors.” Additional language was added to the statement on Wednesday to explain why officials continue to expect prices to fall. Inflation data since the previous meeting in September have hinted at a potential broadening in price pressures and shown that prices for certain items, such as used cars, which saw sharp gains earlier this year, have begun to climb once more.

Mr. Powell also acknowledged more explicitly how increased demand was contributing to supply chain kinks. “Our policy will adapt to, and has already adapted to, changing understandings of inflation, bottlenecks, and the entire supply-side story, which is also partly a demand story,” he said. Higher inflation readings and the actions of other central banks have led bond investors to expect the Fed to raise rates next summer, after it stops buying bonds, and again later in the year.

Mr. Powell has been attempting to strike a balance between assuring investors that the Fed is closely monitoring inflation risks and not appearing so concerned that markets anticipate an even faster shift to tighter money. Global asset prices have been buoyed by the expectation that inflation-adjusted interest rates will remain low. By shifting abruptly, the Fed risks causing new economic or financial stress.

Questions about who will lead the central bank next year loomed in the background of Wednesday’s policy announcements. Mr. Powell will preside over two more policy-making meetings before his term ends in February. President Biden told reporters on Tuesday that he would make a decision “fairly soon” on whether to extend Mr. Powell’s term or appoint someone else to succeed him.