U.S. prosecutors say Conrad Black misled authorities about the value of assets used to secure his bail and have demanded the bail be revoked unless he puts up more of his holdings.

Documents filed Monday with a Chicago court accuse the former media baron of "nondisclosures and misrepresentations" relating to the agreement he made earlier this year to be released on bond.

Black is free, subject to minor travel restrictions, while he awaits a criminal trial in Chicago on 12 charges that he misappropriated millions of dollars from Hollinger International.

Black has pleaded not guilty to all charges. He will forfeit $20 million US if he doesn't show up for the trial, which is set to begin March 5, 2007.

Among other allegations, the U.S. government says Black incorrectly reported the value of a tax lien against a Florida home he put up as collateral.

Prosecutors have demanded that Black put up additional collateral or have his bail revoked.

Black's defence team was given more time to review the court filing. A hearing in the case was put off until June 26.

Bloomberg News quoted Black's U.S. lawyer, Edward Genson, as saying "a lot of things in there [the government filing] are flat out wrong."