New incentives drove an increase in U.S. auto sales in the final week of March, though companies' executives remained split over whether the prolonged market slump has bottomed.

The top five manufacturers all registered smaller year-over-year declines in sales of cars and light vehicles during March, according to reports Wednesday. The improvement left the seasonally adjusted annualized sales rate, or SAAR, well above 9 million for the month, ahead of analysts' estimates and the 9.12 million reported for February.

Executives from General Motors Co. (GM) were the most optimistic about the outlook as the company registered a month-on-month improvement despite a 45% decline compared with March last year.

"We're seeing some stability in the overall SAAR," said GM sales analyst Mike DiGiovanni on a conference call, noting "the first signs of brightening."

His remarks were echoed by Toyota Motor Co. (TM), whose own U.S. sales fell 39% last month. "We are seeing some optimism returning to the market," said group Vice President Bob Carter on a conference call.

Executives at Ford Motor Co. (F) were more cautious, and said that it is too early to call a bottom, though economic indicators suggested improved demand would emerge in around three months. Ford's sales fell 41% last month.

Auto manufacturers said March would have been far weaker but for a late boost in business as a raft of new incentives were rolled out, a process that has continued this week. Average incentives rose 30.1% to a record $3,169 per vehicle last month, according to Edmunds.com, an online retailer.

Mark LeNeve, GM's vice president for North American sales, reiterated that traditional discounts and rebates had less traction than normal, driving companies to innovate. LeNeve added that industry sales typically climbed 21% between March and February, and in fact rose 23% this year. He said any deviation "would have meant that normal seasonal patterns were broken and that the consumer was completely freaked out."

GM faces a 60-day deadline to either develop a new restructuring plan and secure concessions from unions and creditors or else face being pushed into bankruptcy protection by the Obama administration. LeNeve said the March data had no bearing on its ability to meet the timetable.

Auto manufacturers have cut capacity, costs and inventory in the wake of the global slowdown in demand caused by weak economies and tight credit. However, they need to stimulate demand to lift utilization rates and boost cash flow.

In Europe, where sales fell by a quarter in February, there were also signs that incentives had halted the slide last month, with France reporting an 8.1% rise in registrations after a five-month run of year-on-year declines.

GM shares seesawed during its call; it closed down 1 cent at $1.93. Ford cloed up 4.2% at $2.74, and Toyota's American depositary shares rose 7.3% to $67.90.

-By Doug Cameron, Dow Jones Newswires; 312-750-4135; doug.cameron@dowjones.com

(Mike Barris, Jeff Bennett and Kate Linebaugh contributed to this article.)

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