WASHINGTON (Reuters) - Federal Reserve Chairman Ben Bernanke on Tuesday issued a rare warning on the inflationary risk posed by a weak dollar, but said U.S. interest rates were "well positioned" for an economy facing both price pressures and threats to growth.

"We are attentive to the implications of changes in the value of the dollar for inflation and inflation expectations," Bernanke said by satellite to a conference on monetary policy in Barcelona, Spain.

He added that the Fed and the U.S. Treasury were continuing to "carefully monitor" currency market developments.

U.S. officials usually defer on any comment on the value of the dollar to the Treasury secretary, and analysts said Bernanke's remarks were highly unusual.

"It looks like there is behind-the-scenes concern over the dollar and its weakness. He is trying to give that a little bit of verbal support," Kim Rupert, managing director of global fixed income analysis at Action Economics LLC in San Francisco.

The dollar, which has declined steadily in value in recent years against other major currencies .DXY, rose broadly and Treasury debt prices dipped after Bernanke's remarks. The cost of oil, which is priced in dollars, fell.

Bernanke said the Fed's interest-rate cutting campaign -- which has taken benchmark rates to 2 percent from 5.25 percent since mid-September -- and its infusion of billions of dollars into the financial system to ease a credit crunch have helped put a floor under the economy.

"For now, policy seems well positioned to promote moderate growth and price stability over time," he said. "We will, of course, be watching the evolving situation closely and are prepared to act as needed to meet our dual mandate," he said.

Bernanke said until the U.S. housing market stabilizes the economy would continue to face the risk of further weakness. He cited oil prices, which have hit record highs in recent weeks, as another factor weighing on economic growth.

"Activity during the current quarter is likely to be relatively weak," Bernanke said. "We may see somewhat better economic conditions during the second half of 2008."

However, the Fed chief also underscored concerns about inflation from the rising costs of oil and other commodities, although he said that so far they have had only a muted impact on broader prices.

"The pass-through of high raw materials costs to domestic labor costs and the prices of most other products has been limited, in part because of softening domestic demand," he said. At the same time, he warned that "the continuation of that pattern is not guaranteed and will bear close attention."

Bernanke said that if commodity prices stabilized as futures markets predict, there would be a "relatively rapid moderation of inflation," but he said the possibility commodity costs continued to mount presented an important risk to the outlook.

The speech did little to dissuade analysts from the view that the Fed will leave interest rates unchanged at its next policy-setting meeting on June 24-25.

"Bernanke is putting more attention on exchange rates and the dollar and trying to show the Fed's concern about inflation. It clearly looks as if the Fed is going to hold policy steady for a while," said Gary Thayer, a senior economist at Wachovia Securities in St. Louis.

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