Philadelphia Fed Chairman Charles Plosser. (Bradley C. Bower/AP)

A top Federal Reserve official on Tuesday called for the central bank to begin scaling back its massive stimulus program this summer, but acknowledged that it must remain flexible to changing economic conditions.

The Fed has been buying $85 billion in bonds each month since the fall in an effort to lower long-term interest rates and turbocharge the recovery. But recent data suggest that the economy may be gaining its footing, igniting heated debate inside the central bank about how and when to end the buying spree.

The Fed has tied improvements in the labor market to the tapering of its program, though it has not defined what form they should take. In a speech in Stockholm, Philadelphia Fed President Charles Plosser noted that job growth has averaged 208,000 over the past six months — a level that he says warrants pulling back on purchases at the next policy meeting in June. He also said that reducing the stimulus will help ensure that inflation expectations remain low.

“Our extraordinary level of monetary accommodation will have to be scaled back, perhaps more aggressively than some think,” Plosser said.

Plosser opposed the latest round of Fed bond purchases, known as quantitative easing, but he does not hold a voting seat on the central bank’s policy-setting committee. Still, he has been a critical voice in shaping the conversation about how the Fed should return to more conventional policy.

On Tuesday, Plosser said that if the Fed does not scale back its bond purchases, its credibility could be jeopardized, because markets might lose confidence in its willigness to turn off the easy-money spigot. He said the purchases could stop altogether by the end of the year.

But Plosser cautioned that the timeline makes sense only if the economic momentum continues. He emphasized that the Fed could slow down the process or ramp it up, depending on economic conditions. It could even increase the amount of purchases if inflation fell too far or the recovery stumbled.

“We are in uncharted territory in this regard and should be appropriately cautious in specifying too detailed a path that we may not be able to follow,” he said.

Four other top Fed officials — including Chairman Ben S. Bernanke — are scheduled to speak this week, and they could shed more light on the level of support for reducing bond purchases this summer. San Francisco Fed President John Williams said last month that he thought tapering would be appropriate by then.

Officials have stressed that slowing down bond purchases would not signal an imminent rise in the Fed’s target interest rate, which is currently near zero. Most of the Fed’s top officials do not forsee a rate increase until 2015.

“We don’t have Dorothy’s ruby slippers to get us back to Kansas any time soon,” Diane Swonk, chief economist at Mesirow Financial, wrote in a recent research note. “It could be a decade before we return to anything that resembles home when it comes to the Fed’s balance sheet and the conduct of monetary policy.”