Nazi war criminals in the US; The Belarus Secret, by John Loftus. New York: Alfred A. Knopf. 196 pp. $13.95 .

In the aftermath of World War II, while one unit of the newly formed CIA was hunting down Nazi war criminals, another unit was smuggling them into the United States.

At least that's the story as reconstructed by John Loftus, a former trial lawyer for the Justice Department office of special investigations. In defiance of federal law, Loftus asserts, the ''Office of Policy Coordination'' helped obtain visas for a group of Nazi collaborators from Byelorussia - or Belarus - who were believed to have facilitated numerous atrocities by the Third Reich.

According to Loftus, it was all part of a cold war scheme to wage guerrilla warfare in Soviet-occupied Europe, in which the former collaborators were to play a key role. When the project collapsed, however, the Byelorussians quickly settled in and obtained US citizenship - and intelligence agencies protected them from exposure for decades.

Loftus's writing style is on the dry side, but his allegations are not. What gives the book urgency is that, in Loftus's view, comparable abuses within the US intelligence community are still rampant.