U.S. consumer prices tumbled last month due to the sharp drop in energy prices. The underlying inflation rate, however, accelerated at its highest annual rate in over a decade.

The consumer-price index decreased 0.5% in September, the biggest drop since November 2005, the U.S. Labor Department said today.

Excluding food and energy, the CPI advanced 0.2%, the third-straight monthly increase of that size. Economists had forecast a 0.3% decline in the CPI and a 0.2% increase in the core index.

Energy prices fell 7.2%, with gasoline costs dropping 13.5%. An encouraging sign for the winter-heating season, home fuel-oil prices declined 6.1%, though natural gas and electricity prices increased. Food prices rose 0.3%, with vegetable prices jumping 6.6%, the biggest increase since April 2005.

In annual terms, consumer prices increased 2.1% from September 2005. Core prices rose 2.9% in the 12 months ending in September. That’s the highest rate since February 1996 and is well above the Fed’s comfort zone for core inflation of 1% to 2%.

In a separate report, the U.S. Labor Dept. said the average weekly earnings of U.S. workers, adjusted for inflation, increased 1% in September. Average hourly earnings increased 0.2%. Average weekly hours were unchanged.

Meanwhile, U.S. housing starts increased by 5.9% to a seasonally adjusted 1.772 million annual rate in September, the U.S. Commerce Dept. reported.

September’s increase ends three months of declines.

In August, construction fell 4.9% to 1.674 million, revised from an originally reported 6% drop to 1.665 million. Starts fell 4% in July and 6.1% during June.