WASHINGTON (Reuters) - A big rise in energy costs pushed U.S. producer prices up a sharp 0.9 percent last month, but prices outside the volatile food and energy areas rose a slimmer-than-expected 0.1 percent, a government report showed on Tuesday.

The April gain in the overall producer price index, a gauge of prices received by farms, factories and refineries, was the largest since September, the Labor Department said.

It reflected a steep 4 percent rise in energy prices, the biggest rise since October and one that underscored the price pressures stemming from lofty oil prices.

However, the mild gain in the so-called core index - the second consecutive monthly rise of just 0.1 percent - should allay inflation concerns and will be seen as welcome news at the Federal Reserve.

Wall Street economists had expected the closely watched core producer price index to rise a steeper 0.2 percent and looked for the overall index to climb 0.8 percent.

Over the past 12 months, overall producer prices have risen a steep 4 percent. But core prices are up just 1.5 percent, marking a deceleration from the 1.7 percent gain posted in the period through March.

Gasoline prices rose a hefty 12.3 percent in April, the biggest gain since October 2004, while home heating oil prices climbed 13.7 percent and liquefied petroleum gas rose 11.2 percent.

Food costs edged up just 0.1 percent.

The slight rise in core producer prices reflected a nearly offsetting mix of rising and falling prices, including a 0.8 percent drop in passenger car prices, a 1.7 percent increase in the cost of civilian aircraft and a 0.8 percent gain in the price of alcoholic beverages.

Fed officials will keep a close eye on inflation data due out this week after expressing concern that lofty prices for energy and other commodities could spark a wider inflation.

The U.S. central bank raised benchmark overnight interest rates by a quarter-percentage point last week for the 16th straight time, taking them to 5 percent, and said it may need to push rates even higher to address inflation risks.

The department releases the consumer price index on Wednesday. Economists expect overall consumer prices rose 0.5 percent last month, with prices outside of food and energy up a tamer 0.2 percent.

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