Any counterclaim filed in any action brought on behalf of the United States by private counsel retained under may not be asserted unless the counterclaim has been served directly on the Attorney General and the United States Attorney for the judicial district in which, or embracing the place in which, the action is pending. Such service shall be made in accordance with the rules of procedure of the court in which the action on behalf of the United States is pending.section 4241 of this title