General Motors Co (GM.N) on Friday reported a 7.5 per cent increase in U.S. auto sales in August, helped by robust sales of crossovers across its four brands.

GM's smaller rival Ford Motor Co (F.N) reported a 2.1 per cent decline in U.S. auto sales for the month, led by a 11.3 per cent fall in sales of sport utility vehicles. The auto sales decline was the smallest in three months for Ford.

Shares of GM, Ford and Fiat Chrysler Automobiles NV  were up 2 to 4 per cent in morning trading.

"Hurricane Harvey did have an adverse effect on deliveries during the last week of August for every automaker but the key U.S. economic fundamentals remain supportive of strong vehicle sales," GM Chief Economist Mustafa Mohatarem said in a statement.

GM's U.S. vehicle sales rose to 275,552 units in August from 256,429, a year earlier.

Ford reported U.S. sales of 209,897 vehicles in the month, compared with 214,482, a year earlier.

Fiat Chrysler's U.S. vehicle sales fell 11 per cent to 176,033 units.