By Nick Timiraos

WASHINGTON -- Federal Reserve officials signaled plans to keep interest rates near zero for years and said they were studying how to provide more support to a U.S. economy battered by the coronavirus and related shutdowns.

"We are strongly committed to using our tools to do whatever we can and for as long as it takes to provide some relief and stability," Fed Chairman Jerome Powell said Wednesday at a virtual news conference after a two-day policy meeting.

The S&P 500 initially rallied when the Fed released its policy statement after the meeting but later slid to close down 17 points, or 0.53%, at 3190.14. The 10-year Treasury yield fell 0.1 percentage point to 0.728%.

In new projections released Wednesday, all 17 officials who participate in the rate-setting meetings said they expect to hold rates near zero next year, and 15 of them projected rates would stay there through 2022.

"We're not thinking about raising rates. We're not even thinking about thinking about raising rates," said Mr. Powell.

Most officials projected the unemployment rate would average between 9% and 10% during the last three months of the year. That would be down from 13.3% in May, but still well above the 3.5% level of February.

Fed officials projected the economy would contract by anywhere between 4% and 10% this year. Projections for next year were unusually uncertain, with at least one official projecting an additional 1% contraction while most expected growth of around 5%.

Officials made one policy change Wednesday by announcing they would maintain their recent pace of purchases of Treasury and mortgage securities, effectively ending gradual, weekly reductions. They will buy at least $80 billion in Treasurys and $40 billion in mortgage securities, net of maturing bonds, a month.

Mr. Powell played down the positive news from last week's report that the economy unexpectedly added 2.5 million jobs in May, highlighting the potential long-run damage the virus could inflict on the economy by changing consumer and business behavior.

Last week's report "was a welcome surprise. We hope we get many more like it, but I think we have to be honest, that it's a long road," he said.

Even with the gain, there are still nearly 20 million fewer Americans employed than there were in February, and Mr. Powell said it was possible millions of people wouldn't go back to their old job or their prior industry, given the potential for reduced demand for goods or services that require increased human contact.

"It could be some years before we get back to those people finding jobs, " he said.

Mr. Powell lauded the response from officials in Congress and the Trump administration to respond forcefully to the crisis by approving around $3 trillion in various emergency-spending measures but suggested more could be needed as the scale of long-term damage became clear.

"We're doing a fair job of getting through these first few months, more than a fair job. The question, though, is that group of people who won't be able to go back to work quickly -- what about them?" he said.

The central bank cut interest rates to near zero in March amid the pandemic, and officials have raced to put in place a series of programs to lend to businesses, cities and states.

In addition, the Fed has purchased more than $2 trillion in Treasury and mortgage securities since the pandemic sparked a massive flight for safe, cash-like assets in mid-March.

This program has differed from efforts last decade to stimulate the economy by pushing down long-term yields. Instead, officials have said they now are primarily interested in ensuring smooth market functioning.

The Fed entered this week's meeting facing some pressure from markets to clarify the purchases, which it had been reducing every week. The central bank is buying up to $20 billion in Treasurys this week, down from $50 billion six weeks ago, and $375 billion during the week of March 23.

Fed officials have been successful in restoring market functioning, but other factors, such as rising Treasury issuance to finance economic-assistance measures, are threatening to push up long-term yields this summer.

Last Friday's employment report, for example, sent the 10-year Treasury yield rising by 0.2 percentage point, to 0.94%, though yields had reversed that rise by Wednesday.

Potential confusion about the Fed's long-run bond-buying intentions, together with the coming surge of Treasury supply, "invited the kind of selloff we had Friday," said Robin Brooks, chief economist at the Institute of International Finance. "We don't want financial conditions to tighten on one noisy data point. We just don't."

The Fed's goal should be to maintain stable long-term yields without getting too specific about how the central bank will do that, said Mr. Brooks.

With rates unlikely to go lower, another major element of officials' evolving policy stance will center on how to communicate their long-run intentions, using what is known as forward guidance.

Mr. Powell said officials continued discussions this week about whether to tie their rate plans to certain economic outcomes, such as inflation returning to 2% and unemployment returning to its recent low levels.

Alternate approaches would tie rate changes to specific dates, and potentially cap yields on certain Treasury securities by purchasing them as needed to reinforce their rate guidance.

A separate policy review, which was interrupted by the current economic crisis, had built support within the Fed to seek modestly higher inflation after periods in which inflation fell below the 2% target. Officials are still working on that review, which could shape how they define their plans.

The Fed's actions have buoyed financial markets for now, with stock markets recently returning to positive territory for the year. But economic crosscurrents have made it hard to diagnose the underlying damage inflicted by the virus.

Waiting until September to unfurl any new strategy might give the Fed a better idea of how to calibrate it, in part because of the extremely unusual nature of the current shock. By then, they could know more about any additional economic relief from Congress, the prospect for vaccines and other information about how the virus spreads as commercial activity resumes.

"They want to keep their powder dry in this circumstance because they really haven't any experience where they're going to see a surge in economic activity but not an economic boom," said Randall Kroszner, a former Fed governor.

David Wilcox, a former senior Fed economist who is now at the Peterson Institute for International Economics, said, "There is a place for the Fed eventually to engage in some housekeeping to tidy and sharpen up the common understanding of how those policies will be conducted in the longer term, but the urgency is rather low at this point."

Write to Nick Timiraos at nick.timiraos@wsj.com