"We continue to be of the opinion that there is a high risk of flight," said ministry spokesman Folco Galli.
The chance was too great for the government to accept bail or other security measures in exchange for the filmmaker's release, Galli said.
The 76-year-old Oscar-winning director of The Pianist is wanted by U.S. authorities for having sex with a 13-year-old girl in 1977. Polanski fled the United States in 1978 after pleading guilty to one count of unlawful sexual intercourse with the underage girl. The Swiss have said that U.S. authorities have sought Polanksi's arrest since 2005. He was arrested in Switzerland as he arrived in the country for a film festival on Sept. 26.
Washington has yet to file a formal request seeking extradition, Galli said, but the U.S. probably wouldn't need its entire 60-day period to submit all documents.
"I assume this is a priority case in the United States," Galli said.
He also added that Polanski would likely remain in prison for the duration of his case, which the Federal Criminal Court said it will rule in the "next weeks."