Market turmoil failed to put the brakes on US auto sales in August, automakers said Thursday as the Big Three US automakers -- General Motors, Chrysler and Ford -- all posted strong gains.

"Consumers are being cautious and appropriately so, but they are not retrenching," Don Johnson, vice president for GM US sales, said in a conference call.

GM is "cautiously optimistic that we'll see US economic growth improve in the months ahead," Johnson said, noting that while consumer confidence has taken a hit in recent weeks, retail sales actually improved.

Strong retail sales helped GM boost deliveries 18 percent in August to 218,479 vehicles last month and lower-margin fleet sales also posted gains, the automaker said.

GM said it is "closely monitoring consumer sentiment and other economic indicators" but has not changed its forecast for overall 2011 industry sales, which it expects to be in the "low end" of the 13-13.5 million vehicle range.

"We've got a historically very low industry and still a lot of pent-up demand up there," Johnson said. "All indications are to us that the industry is going to slowly grow this year."

Americans bought between 15 and 17.8 million vehicles a year from 1994 to 2007. The 2008 crash knocked sales down to levels not seen since the recession of the early 1980s.

Chrysler, which like GM was forced into a government-backed bankruptcy restructuring in the wake of the crash, posted its best August since 2007 as US sales rose 31 percent to 130,119 vehicles.

"In spite of a volatile market, Chrysler managed to significantly outperform the industry again in August," Reid Bigland, head of Chrysler's US sales, said in a statement.

"Last month also represented our 17th consecutive month of year-over-year sales growth, and on the back of our 16 all-new or significantly-refreshed products I see that trend easily continuing into the fall."

Ford showed more modest sales gains of just 11 percent to 175,220 vehicles as it announced plans to boost fourth quarter production nine percent from 2010's depressed levels.

"Ford's fuel-efficient cars, crossovers and trucks continue winning over consumers in the marketplace," Ken Czubay, Ford vice president for US sales, said in a statement.

Japanese automakers Honda and Toyota, however, continued to be adversely affected by supply shortages in the wake of the devastating March 11 earthquake and tsunami.

Toyota's US sales fell 13 percent to 129,483 vehicles in August but are expected to improve in the coming weeks as dealers are able to fill their lots.

"Our supply outlook continues to improve as North American production will return to 100 percent of normal levels this month," said Jeff Bracken, vice president of Toyota Division sales.

Honda sales dropped 27 percent to 82,321 vehicles in August amid supply shortages.

"The production plan for most of our models returned to 100 percent by late August at all of our North American factories," said John Mendel, executive vice president of sales for American Honda.

"A greater selection of vehicles is beginning to arrive at dealers this month as the company continues to recover from temporary supply disruptions."

Overall industry sales declined sharply in the second half of August, according to an early analysis of real-time transactions by JD Power and Associates.

"With the economic woes, summer vacations and Hurricane Irene taking center stage, August may be a lost month for vehicle sales, but the slight increase in the retail selling rate from last month is still a step in the right direction," said Jeff Schuster, executive director of global forecasting at JD Power.

"Marketing and incentive focus has already shifted to September with the upcoming Labor Day weekend, so with improved inventory, the sales pace could show marked improvement next month."

JD Power forecasts that August sales will slip to a seasonally adjusted annualized rate of 11.9 million vehicles, down from 12.2 million in July.

Final industry sales figures were expected later Thursday.