LONDON  Lawyers for Julian Assange, the founder of the WikiLeaks antisecrecy group, said on Tuesday that they would oppose his extradition to Sweden because he might subsequently face “illegal rendition” to the United States, risking imprisonment at Guantánamo Bay, Cuba, or even the death penalty.

They made the assertion in defense documents released after Mr. Assange made a brief appearance in a British high-security court for a largely procedural hearing concerning his resistance to demands for his extradition to Sweden, where he has been accused of sexual misconduct.

The documents for the first time publicly named the two WikiLeaks volunteers who have accused Mr. Assange of forcing them to have sex with him without a condom in Sweden last August, in one case while the woman was asleep. In keeping with Sweden’s policy of anonymity for those involved in such cases, they had been referred to only as Ms. A and Ms. W.

Karin Rosander, a spokeswoman for the Swedish prosecutors’ office, could not immediately say whether the disclosure of the women’s names was a crime, but she said that possibility would be investigated. Jennifer Robinson, one of Mr. Assange’s London lawyers, said that the inclusion of the women’s names in the defense documents was an oversight that would be corrected.