WASHINGTON — The Federal Reserve chairman, Ben S. Bernanke, said Monday that recent declines in unemployment were likely to continue only if the economy grew more quickly.


In a speech that sought to deflate by turns both optimism and pessimism about the labor market, Mr. Bernanke said the Fed’s efforts to stimulate growth were gradually reducing unemployment, but that the scale and duration of the problem could leave lasting scars on the economy.


“Recent improvements are encouraging,” he said. However, he continued, “millions of families continue to suffer the day-to-day hardships associated with not being able to find suitable employment.


Mr. Bernanke said he was particularly concerned about the unusually large share of the unemployed who have been unable to find work for six months or more. Roughly 40 percent of the unemployed fall into this category, according to government statistics, compared with 25 percent after other recent recessions.


In addition to the heavy toll on workers and families, he noted that “because of its negative effects on workers’ skills and attachment to the labor force, long-term unemployment may ultimately reduce the productive capacity of our economy.”


Investors are hungry for indications about the direction of Fed policy, and major stock market indexes rose after Mr. Bernanke’s speech, which was delivered to a conference of business economists meeting in Arlington, Va. The Standard & Poor’s 500-stock index was up 1 percent.


But analysts who follow the central bank said the speech contained no obvious indications about Mr. Bernanke’s plans.


The central bank will conclude its current round of asset purchases by the end of June. Some Fed officials want to begin another round in the aftermath, focused on mortgage-backed securities, while others oppose new actions. Mr. Bernanke and his principal advisers have remained tight-lipped on the issue.


The speech did offer a clear defense of the Fed’s ongoing and expansive campaign to promote growth. Critics, including some Fed officials, argue that the central bank’s policies are likely to trigger inflation by attempting to stimulate growth when the economy already is running near capacity.


Unemployment remains high, at 8.3 percent, but those concerned about inflation say the Fed’s policies cannot help many of those people because they lack skills.


Economists describe this kind of problem as “structural” unemployment, rather than cyclical unemployment, which is caused by general economic weakness.


Mr. Bernanke said Monday that structural unemployment may have increased in recent years, but that high unemployment was mostly cyclical.


“The continued weakness in aggregate demand is likely the predominant factor,” he said. “Consequently, the Federal Reserve’s accommodative monetary policies, by providing support for demand and for the recovery, should help, over time, to reduce long-term unemployment as well.”


Mr. Bernanke outlined several reasons for this conclusion. He said that people who lost work recently had not increased their advantage in finding new jobs over the long-term unemployed, as might be expected if the latter group were languishing because they lacked relevant skills. And he noted that hiring generally remained weak across the economy, again suggesting broad causes rather than industry-specific roots.