WASHINGTON, D.C. ? A federal judge blocked Vice President Dick Cheney’s former chief of staff Friday from using some classified material to help with his defense in the CIA leak case.

Former White House aide I. Lewis “Scooter” Libby has been granted access to a number of classified documents to prepare for trial, but U.S. District Judge Reggie B. Walton denied Libby’s request for an unspecified number of other documents.

“The documents and information at issue are extremely sensitive and their disclosure could cause serious if not grave damage to the national security of the United States,” Walton wrote in a ruling that sides with prosecutors and intelligence officials who opposed turning them over.

Libby faces charges of obstruction of justice and lying about how he learned the identity of CIA officer Valerie Plame and what he told reporters about her.