NEW YORK (CNNMoney.com) -- The U.S. labor market showed increasing signs of weakness last month as the number planned job cuts topped 100,000 in September, according to a report released Tuesday.

The survey, published by the global outplacement consultancy group Challenger, Gray & Christmas, revealed that planned job cuts reached 100,315 last month, up 54 percent from August and up 40 percent from the same period last year.

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"The top reason provided by job-cutting employers during the month was a downturn in demand," said John A. Challenger, chief executive officer of Challenger, Gray & Christmas. "We probably would not see permanent job cuts if the downturn were considered temporary."

While planned job cuts soared in September, layoffs are still down 18 percent so far this year, compared to the same nine-month period in 2005.

Challenger, Gray & Christmas said the majority of payroll cutting came from the automotive sector, which announced 33,745 cuts, while suppliers to automakers Ford (Charts), General Motors (Charts) and DaimlerChrysler (Charts) were particularly hard hit.

Softness in the housing sector has sparked job cuts at such firms as the homebuilder Pulte Homes (Charts), according to the survey, while both the telecommunications, computer industry and consumer product companies all reported more planned job cuts last month.

September typically marks the start of the heaviest job-cutting part of the year, Challenger said, noting that he would not be surprised if another 30,000 cuts were announced before the end of the year.

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