BEIJING (REUTERS) - China Southern Airlines said it reported four middle-ranking executives to authorities after an internal audit uncovered irregularities as China steps up its anti-corruption campaign.

A spokesman for China's biggest airline by fleet size declined to identify the executives, now under investigation by the police. The spokesman said on Monday he could not comment on the nature of the irregularities.

The airline's comments came after China's official Xinhua news agency reported on Nov 30 that as many as 10 executives at China Southern were taken away by police last week as part of the anti-graft drive. China Southern's spokesman said on Monday only four company executives are currently under investigation.

China's new leadership has made fighting corruption a top theme and has vowed to pursue what it called high-flying"tigers" as well as "flies" to assuage rising public anger over the scale of graft in the world's second-biggest economy.

The Xinhua report on China Southern identified executives involved in sales and marketing positions as being the subject of the probe. China Southern's spokesman declined to name any of those involved nor comment on their positions.

The anti-corruption campaign has led to the detention of some senior government officials as well as senior executives in major state firms, including PetroChina Co Ltd .

Last month Cosco Holdings, China's largest bulk shipping firm, said that company vice president Xu Minjie was placed "under investigation by the relevant authorities", phrasing used in China to describe corruption investigations.

China Southern's Hong Kong-listed shares slipped 0.65 per cent in early afternoon trade, trailing a 0.11 per cent gain in the broader market.