WASHINGTON (AP) _ A female KGB agent involved with a Marine security guard suspected of espionage was employed by the U.S. embassy in Moscow, according to a report published today.

The Soviet woman worked at the embassy while Sgt. Clayton J. Lonetree served there between Sept. 24, 1984, and March 10, 1986, said The Washington Times, quoting sources close to the case.

Lonetree is suspected of helping the Soviet spy agency plant electronic listening devices within embassies in Moscow and Vienna, where the guard was transferred last March, according to the newspaper.

The State Department declined to comment on the reported penetration of the embassy by a KGB agent beyond confirming the detenton of the Marine guard.

''Security at the embassy in Moscow, for lots of reasons, is under constant review,'' spokeswoman Phyllis Oakley said.

Sources told The Associated Press on Saturday that Lonetree, 25, of Chicago, engaged in a sexual relationship with the woman and later admitted to U.S. officials in Vienna that he passed secrets to her.

He has been in detention in Quantico, Va., since Dec. 31 awaiting a military hearing to determine charges, said Marine Corps spokesman Lt. Col. John Shotwell. The hearing will be held in two or three weeks, he said.

Four charges could be brought against the Marine: espionage, conspiracy to commit espionage, failure to report contact with citizens of Communist-contro lled nations and unauthorized removal and disclosure of classified information, Shotwell said.

A conviction of espionage can bring the death penalty under the Uniform Code of Military Justice.

Lonetree had top secret clearance and had access to classified documents, the spokesman said.