The United States yesterday expelled an assistant military attache at the Soviet Embassy here in response to what it called "the unacceptable Soviet position" in a statement Monday on the killing of U.S. Army Maj. Arthur D. Nicholson Jr. in East Germany last month.

Assistant Secretary of State Richard R. Burt informed acting Soviet Ambassador Oleg Sokolov that Lt. Col. Stanislav Ivanovich Gromov is persona non grata and has seven days to leave the United States.

Burt demanded clarification of the Soviet position and insisted that Moscow abide by the agreement reached April 12 between the U.S. Army commander in Europe, Gen. Glen K. Otis, and his Soviet counterpart, Gen. Mikhail Zaytzev, a State Department statement said.

The State Department said the Soviet side agreed then to take measures to prevent a repetition of the incident by prohibiting the use of force or weapons against members of the U.S. military liaison in East Germany, as provided in a 1947 agreement.

Monday, however, the Soviet Union issued a statement reserving the right to deal with any "intruder" on an "intelligence mission" according to Soviet military manuals rather than the 1947 accord.

Burt reiterated to Sokolov the U.S. position that the Soviets should apologize for Nicholson's death and compensate his family, the State Department statement said. Otis will seek further discussion with Gen. Zaytzev to prevent other incidents of violence, the department said.

A senior State Department official said the choice of Gromov for expulsion had been on the advice of the Pentagon, and noted that the colonel had been considered "very active" in his duties here. He refused to comment further.

The official hinted that Secretary of State George P. Shultz would take up the matter with Soviet Foreign Minister Andrei Gromyko when the two meet in Vienna May 14.