Roman Polanski has been denied bail from a Swiss prison where he is being held for possible extradition to the United States on an underage sex charge.

The Swiss justice ministry refused Polanski's request because of the risk he would flee the country, Folco Galli, a spokesman for the ministry, told the Associated Press.

"We continue to be of the opinion that there is a high risk of flight," he said.

The appeal was separate from another legal challenge against his arrest submitted by Polanski's lawyers last week. Polanski is expected to be held in custody throughout that appeal and any subsequent challenges from either side.

US authorities have being trying to secure Polanski's extradition since he fled in 1978 while awaiting sentencing after admitting have sex with a 13-year-old girl.

Under Swiss law the US has 60 days to file a formal extradition request. That request must be examined by the justice ministry, and if approved can be appealed against in a number of courts. That appeal hearing is expected to start in the coming weeks.

Galli confirmed that Washington has yet to file a formal request seeking extradition. But he added: "I assume this is a priority case in the United States."

Swiss legal experts say Polanski's chances of release are minimal, because he is not Swiss and he already jumped bail.

Polanski was accused of raping the girl, after plying her with champagne laced with drugs.

He initially faced six charges including rape by use of drugs, child molesting and sodomy.

He pleaded guilty to the lesser charge of unlawful sexual intercourse. In exchange, the judge agreed to drop the remaining charges and sentence him to prison for a 90-day psychiatric evaluation.

However, he was released after 42 days by an evaluator who deemed him mentally sound and unlikely to offend again.

The judge responded by saying he was going to send Polanski back to jail for the remainder of the 90 days and then he would ask Polanski to agree to a "voluntary deportation".

Polanski fled the US on the day he was scheduled to be sentenced to the additional time.

Polanski's arrest sparked contrasting reactions. France and Poland have called for his release, but support for him in Hollywood provoked an angry backlash.