US consumer confidence fell slightly in September as Americans grew more wary about the outlook in coming months, a closely watched report released Tuesday showed.

The Conference Board said its consumer confidence index fell to 79.7 in September, down from a revised 81.8 in August.

Though a decline was expected for September, the figure was weaker than the 80.0 consensus estimate.

"This is in line... with buyers awaiting a battle among lawmakers over raising the Treasury debt ceiling that could threaten a government shutdown," said Nate Kelley of Moody's Analytics.

The Present Situation Index rose to 73.2 in September from 70.9 in August.

The Expectations Index, indicating consumers' views on the outlook six months into the future, dived to 84.1 from 89.0 last month.

"Consumer confidence decreased in September as concerns about the short-term outlook for both jobs and earnings resurfaced, while expectations for future business conditions were little changed," said Lynn Franco, the Conference Board's director of economic indicators.

"While overall economic conditions appear to have moderately improved, consumers are uncertain that the momentum can be sustained in the months ahead."

Consumers' outlook on jobs and incomes in the coming months darkened.

The number of consumers expecting more jobs fell to 16.9 percent from 17.5 percent in August, while those anticipating fewer jobs rose to 19.7 percent from 17.2 percent.

Income expectations were sharply lower. Those expecting their incomes to increase tumbled to 15.4 percent from 17.5 percent.

The concerns about lower incomes could crimp consumer spending, the engine of US economic growth.

The decline in expectations "also gibes with our call for a relatively weak holiday shopping season this year," said Moody's Kelley.