Average spending by carmakers on consumer incentives in the US edged up last month, an industry research group said on Thursday, according to Reuters.

Incentives across the industry averaged $US3,046 in May, up from $2,994 per vehicle in April, the news agency said, citing Autodata's monthly report.

General Motors once again led the industry with an average of $4,325 per vehicle in May, an increase of $96 from April for the firm, which posted a 6.5% increase in US sales last month, the report said.

DaimlerChrysler's Chrysler division, which posted 5% US sales gains last month, spent an average of $3,966 per vehicle on incentives in May - the carmaker's incentives fell slightly from $4,030 in April, Reuters said.

According to the report, Ford, which saw its US sales decline 3.1% for May, was third in incentives spending with an average of $3,515 - down a bit from April.

Reuters noted that the US car industry posted its strongest sales in nine months in May, boosted by heavy discounts.

The average Big Three incentives edged up to $3,991 per vehicle in May from $3,961 in April, the report added.

Reuters said average spending by Asian brands totalled $1,529, a 9% increase from April, while European brands totalled $2,643 per vehicle in May, a 17% increase from the previous month.