Defense lawyers say Oliver L. North cannot be prosecuted for lying to Congress because the former presidential aide was never warned that making misleading statements about covert operations was a crime.

Citing false statements by government officials about other covert operations, North's defense lawyers contended Tuesday that Iran-Contra prosecutors were bringing novel charges against their client.The defense said that until North's indictment March 16 with three other Iran-Contra defendants, "no executive branch official had ever been prosecuted based on unsworn statements to Congress."

North did not receive a "fair warning that an executive branch official or a member of his staff would be guilty of a crime if the truth were misstated in an unsworn communication to Congress," the defense said.

The defense filed a series of motions to dismiss various charges that North, a former National Security Council aide, misled House committees in 1985 and 1986 by denying involvement in efforts to arm the Contras or raise money for the rebels fighting Nicaragua's Sandinista government.

The defense noted that none of the statements made by North were under oath, including remarks he made during an "informal off-the-record discussion" with members of the House Permanent Select Committee on Intelligence in 1986 in the White House Situation Room.

The other statements were written responses to queries made by various House committees about reports of covert assistance to the Nicaraguan rebels.

An indictment returned last March charges North, former national security adviser John M. Poindexter and arms dealers Albert Hakim and retired Air Force Maj. Gen. Richard V. Secord with conspiring to defraud the government by diverting U.S.-Iran arms sale profits to the Contras.

U.S. District Judge Gerhard A. Gesell has ordered separate trials for each defendant. North is set to be tried first, but no trial date has been set.

North's lawyers noted there was no prosecution of Secretary of State John Foster Dulles for falsely assuring lawmakers in 1958 that the government was not intervening in Indonesia even though the CIA was supporting rebels fighting President Sukarno.

Likewise, the defense said, there were no prosecutions arising from false statements by the State Department after a U-2 spy plane was shot down over the Soviet Union in 1960, the defense noted.

There may be disagreement "as to whether or to what extent it is proper for government officials to misstate or conceal the truth where a truthful statement would jeopardize intelligence gathering or covert operations or otherwise compromise national security," North's lawyers said.

"Until this case, however, there was no notion that such a false or misleading statement could be criminal simply because the statement was made directly or indirectly to a member of Congress," the defense said.

"The executive branch communicates with Congress on a continual basis about countless matters relating to the business of government," the defense said.

North's lawyers said the provision under which he was charged only applied to statements made in connection with Congress' administrative functions and "could not have been intended to reach such communications."

If it were applicable, "it would allow prosecution of executive branch officials for misrepresenting their intentions or concealing material facts," defense lawyers said.