Lawyers for accused spy Ronald W. Pelton argued today that their client's alleged confession to FBI agents should not be admitted as trial evidence because the agents used "improper direct and indirect promises" to get Pelton to talk to them.

Documents filed in U.S. District Court here state that the two FBI agents who interviewed Pelton shortly before he was arrested Nov. 25 used "psychological coercion" and "fraud, deceit and trickery" to obtain statements from him.

Pelton's lawyers have said they believe the government's case will collapse if prosecutors are unable to use Pelton's statements to the FBI in his trial. However, the government has had marked success in arguing for the legality of similar statements in other recent spy cases.

Pelton, 44, a former communications specialist with the highly secret National Security Agency, is alleged to have told the FBI agents that he sold extremely sensitive U.S. intelligence information to the Soviet Union from 1980 to 1985. Federal prosecutors charge that he received cash payments totaling at least $35,000 for information he provided during trips to the Soviet embassies in Washington and Vienna.

Fred Warren Bennett, Pelton's court-appointed lawyer, said today he could not comment directly on what promises were made to Pelton in exchange for his cooperation with the agents.

But during a detention hearing in November, defense lawyers suggested that the agents led Pelton to believe that he could escape prosecution by cooperating with them and agreeing to work as a double agent for the United States.

Under cross-examination by defense lawyers then, Agent David E. Faulkner acknowledged that during five hours of talks in an Annapolis hotel room, he and agent Dudley Hodgson told Pelton that U.S. intelligence agencies might want to send him overseas.

However, Falkner said, the agents told Pelton that the purpose of the trips would be for Pelton to point out the places where he allegedly met with Soviet KGB agents. Faulkner denied that Pelton was asked if he would do counterintelligence work targeted at the Soviet Union.

Pelton, who left NSA in 1979, had clearance for highly classified information during his 14 years with the agency. The FBI has said Pelton was identified as a spy through information provided by former KGB official Vitaly S. Yurchenko, who defected to the United States last year, then returned to the Soviet Union.

In their motion today to exclude Pelton's statements to the FBI as evidence, defense lawyers maintained that the agents violated Pelton's constitutional rights during the discussions that led up to his arrest. They argued that he was denied his right to have an attorney present during the interviews, and that he was not read his rights before he was asked to make statements to the agents.

Pelton, who is being held in the Anne Arundel County Detention Center, is scheduled to go to trial March 24. A ruling on today's motion is not expected until next month.

In the last year, some other accused spies have failed to get the courts to exclude from evidence statements they made to FBI agents about their activities. Among them are convicted spy Arthur Walker, whose statements formed the foundation of the government's case against him; Jerry Alfred Whitworth, who stands trial next month on charges of spying for the Soviets; and Larry Wu-Tai Chin, a retired CIA analyst accused of spying for the Chinese for more than 30 years.

Pelton's lawyers are also trying to get any wiretap evidence the government may have on him excluded from trial. The government has not revealed what evidence it has against Pelton beyond his statements to the FBI, but Bennett said he has been told informally by prosecutors that they intend to introduce recorded telephone conversations.