Independent counsel Lawrence Walsh filed a motion Thursday to dismiss conspiracy and theft charges against fired National Security Council aide Oliver L. North in the Iran-Contra case.

Walsh said he was compelled to take the step due to the large amount of classified material that would be disclosed at North's trial, scheduled to begin Jan. 31.If U.S. District Judge Gerhard Gesell approves the motion to dismiss the charges, there still would be a dozen criminal charges remaining against North, including allegations that he obstructed inquiries and made false statements in the Iran-Contra affair.