Swiss pharmaceuticals giant Roche and banking titan HSBC are among foreign firms being probed in South Korea for alleged involvement in illegal sales of swine flu treatment, officials said Friday.

The Korea Food and Drug Administration (FDA) said it raided the Seoul office of Tamiflu maker Roche Holdings on Wednesday and seized computer files and other documents.

"Roche is under investigation as part of our probe into illegal Tamiflu sales and purchases," FDA spokesman Ahn Man-Ho told AFP.

The raid followed an investigation which began in May into local hospitals, drugstores and companies accused of selling or stocking up Tamiflu illegally, he said.

South Korean laws ban large-scale drug purchases by non-medical professionals without doctors' prescriptions. Violators could face up to five years in prison.

The agency said Roche is suspected of helping the Korean units of London-based banking giant HSBC Holdings and Novartis of Switzerland to buy large stocks of Tamiflu.

HSBC and Novartis were found to have bought enough for more than 5,900 people, it said. "We have already asked prosecutors to start a criminal investigation into HSBC and Novartis," Ahn said.

The agency has also raided the offices of other foreign firms in Seoul, he said without giving details.

The investigation comes amid mounting concern about the rapid spread of the (A)H1N1 virus.

This week South Korea raised its flu alert from "orange" to the highest "red" level, with 48 deaths attributed to swine flu. Some doctors point out the death rate is no higher than during a normal flu season.

South Korea has enough Tamiflu to treat 5.3 million people, just over 10 percent of the population. It will secure treatment for more than 10 million by the end of the year.

HSBC's Seoul branch spokeswoman Jina Hwang declined comment since the case is under investigation.

But she said the bank issued a statement in September, saying it had secured enough Tamiflu for its Korean staff under guidelines set by its headquarters.

The bank said at that time it bought Tamiflu based on a contract with a local medical institute to obtain prescriptions.