US consumers tightened their purse strings unexpectedly last month, breaking a seven-month winning streak, government data showed today. 

Shoppers bought fewer cars and spent less on fuel, groceries and building supplies while buying less online as well, according to the Commerce Department. 

The September slump meant momentum waned at the end of the third quarter and could be a worrying development for the world's largest economy. 

The US consumer is almost single-handedly sustaining the US expansion as the economy in the rest of the world slows and President Donald Trump's trade wars eat into US exports, business investment, manufacturing and agriculture. 

The September dip in sales looked bigger after August's numbers were revised upward, however. 

Compared to August, total retail sales fell 0.3% to $525.6 billion, the first decline since February. 

Economists had instead expected a 0.3% increase, as shoppers moved to buy at lower prices ahead of looming tariff increases on Chinese goods. 

Compared to September of last year, sales were up 4.1%. 

Sales of cars and auto parts slumped 0.9% while petrol stations were down 0.7%. 

But, even when these volatile categories are excluded, the picture was not much brighter, with sales flat for the month.

Ian Shepherdson of Pantheon Macroeconomics said other data showed September had been stronger than it appeared, meaning the latest numbers were likely to be revised upward.

Upward revisions occur more frequently than downward ones for retail sales, he said, "and today's numbers look like prime candidates to be moved higher."

However, sales growth in the fourth quarter was likely to continuing slowing, falling back in line with slower growth in after-tax income, he said.