NEW YORK (CNN/Money) - U.S. chain store sales rose during the first three weeks of October as cooler weather put shoppers in the mood to buy winter clothing, but sales slowed a bit last week, two industry reports showed Tuesday.

Retailers are worried that the critical holiday period, which accounts for half of their year's sales and profits, will fall victim to sinking consumer confidence and shipping delays stemming from the 10-day lockout of West Coast ports.

If merchants were biting their nails before, the latest report on consumer confidence Tuesday gave them even more reason to worry, as the index posted its biggest drop since 1993.

The Conference Board said its consumer confidence index plummeted to 79.4 from a revised 93.7 in September, the fifth straight decline for the index and far worse than analysts expected.

Consumer attitudes are a key measure of the nation's economic health and for retailers, since consumer spending fuels two-thirds of the U.S. economy. Consumers have been credited with pulling the U.S. out of a recession earlier this year amid sluggish business spending.

Still, Americans have not let tough economic news stop them from taking advantage of low interest rates and zero-percent financing deals to buy houses and cars.

October's sales results so far have proven that consumers, though behaving cautiously, are not quite padlocking their wallets.

U.S. chain store sales rose 1.4 percent in the three weeks ended Oct. 26 compared with the same period in September, Instinet Research said in its weekly Redbook report.

"Sales pulled back in the third week," the report said. "However, the tone of business remains firm and October continues to outpace September."

The Redbook average is compiled from a sample of general merchandise retailers representing about 9,000 stores. Same-store sales measure revenue at stores open at least a year.

Separately, Bank of Tokyo-Mitsubishi reported a 2.6 percent sales increase for the week ended Oct. 26, compared with the same period a year ago. But last week's sales fell 1.9 percent compared with the previous week, reflecting tough comparisons with the week leading up to the Columbus Day holiday.

BTM forecasts total October sales will increase just 1.5-2 percent from a year earlier.