The Justice Department yesterday announced its intention to jump into a family dispute that has international implications.

Thomas DeCair, chief spokesman for Attorney General William French Smith, called a press briefing to say the department wants to make sure that 13-year-old Walter Polovchak, who was granted political asylum when his parents returned to the Soviet Union, doesn't have to join them against his will.

In doing so, he said the department was rejecting a proposed agreement over a Chicago federal court challenge to the asylum and is planning to ask a state appeals court in Illinois to protect the youngster's rights as well.

Harvey Grossman, an American Civil Liberties Union lawyer representing the youth's parents in the federal asylum challenge, denounced the Justice Department's intervention yesterday as a "Cold War tactic" that amounted to an attempt to "improperly influence" the Illinois court.

DeCair denied there were any political overtones to the department's action. He said Deputy Attorney General Edward C. Schmults decided on his own to kill the settlement that would have prevented Justice from taking an interest in the asylum case.

The conservative publication, Human Events, however, took some credit in its current issue for turning the department around. It said Schmults "has been taking a more conservative line" since it carried an article critical of the department's liberal policies a few weeks earlier.

The young Polavchak became a celebrity in July, 1980, when he ran away from his home in Chicago after hearing that his parents planned to return to the Ukraine. After he was found he was placed in temporary custody of the state, and was later granted asylum on the grounds he was a Baptist and might be persecuted if returned to the Soviet Union.

A court battle has been going on since. The parents fought for custody in the state court and against the asylum in the federal court. Although they have returned to the Soviet Union, lawyers representing them are actively pursuing the cases.

Earlier this month, an assistant U.S. attorney in Chicago reached a tentative agreement with the ACLU's Grossman that the government would express no interest in the case. DeCair said that Schmults rejected it immediately after hearing about it earlier this month.

In a letter to the Illinois appeals court, the department said it planned to file a friend-of-the-court brief in the dispute over whether the youth should stay with a foster family while his custody battle is settled.

"The United States supports the grant of asylum to Walter Polovchak, and in its brief will urge the court in its decision to reach no conclusion that would allow Walter Polovchak to be returned to the Soviet Union against his will," the letter said.