During the same period that he was allegedly spying for Hanoi, U.S. Information Agency employe Ronald L. Humphrey was approached by a Soviet espionage agent for information, but immediately informed authorities about the incident, Humphrey's lawyer said yesterday.

The agent, Vladimir I. Alekseyev, subsequently was expelled from the United States. He ostensibly had been working here as a reporter for the Soviet news agency Tass.

Humphrey's lawyer, Warren L. Miller, told U.S. District Court judge Albert V. Bryan Jr. in Alexandria yesterday that after Humphrey's initial contact with Alekseyev the FBI used the USIA employe to help gather information about the Russians.

The Russians's expulsion on Feb. 6, 1977, was explained at the time by the State Department as a response to the expulsion of Associated Press correspondent George Krimsky from Moscow a few days earlier.

In court yesterday, Miller called the State Department explanation a guise. he also said the incident proved his client's "clear intend to assist his government and report possible compropises of security."

The State Department said yesterday that it does not know of any connection between Alekseyev and Humphrey. Alekseyev held a position here in the Tass bureau that was comparable to that held by Krimsky in Moscow.

The FBI and government prosecutors declined comment on Miller's allegation.

Miller said he based his statement on official USIA documents given him by the government. He mentioned the Alekseyev matter yesterday while asking Bryan for an order requiring the FBI to hand over two files on the incident.

Humphrey first met Alekseyev on election eve 1978 at a party in the National Press Building, according to his attorney. Humphrey was immediately suspicious of the Russian, who seemed to know too much about him and his background, Miller said.

As a USIA watch officer, Humphrey has a "Top Secret" clearance and access to classified cables, some of which he is accused of stealing for his codefendant David Truong during 1976 and 1977.

Miller told the court that Humphrey informed USIA security of his overturesa from Alekseyev almost as soon as they were made.

According to Miller, Humphrey was told to stay in touch with Alekseyev and to write a report to USIA officials on any contacts he had with him. Alekseyev made several phone calls to Humphrey and then, in December 1976 had a long lunch - approved by Humphrey's superiors with him at the Old Ebbitt Grill.

By that point, Miller said after the court hearing, "The FBI knew and informed Humphrey that Alekseyev was connected with Soviet security services and engaged in Soviet intelligence activities in the U.S."

In January 1977 Humphrey was de-briefed for several hours about all his contacts with the Russian. He continued to receive calls from Alekseyev, however, until Alekseyev was expelled.

"This whole affair," said Miller, "is going to be a vital part of our defense. (Humphrey's) actions are completely an espionage conspiracy."

Bryan did not rule yesterday on the Humphrey motion for the FBI documents or on numerous other motions by defense lawyers. His ruling on those motions and on the courtroom use of controversial warrantless wiretap evidence approved by Attorney General Grittin B. Bell and President Carte is expected within a week.