The nation's inmates are serving more time behind bars, particularly for violent crimes, a fact that helped push the U.S. prison population above 1.2 million in 1997, Justice Department officials say.

The longer sentences, coupled with a fairly constant stream of admissions, produced the slow but steady 5.2 percent growth over the decade to 1,244,554 prison inmates last year, the department's Bureau of Justice Statistics reported Sunday. More than a half-million others were serving shorter sentences or awaiting trial in jails.With stable admissions, "the prison population growth in the 1990s has been primarily driven by the increasing lengths of stay - fewer inmates leaving," bureau statistician Allen J. Beck, the report's co-author, said in an interview.

"Increased time served, particularly for violent crimes, is a product of tougher parole boards and such measures as longer minimum sentences and truth-in-sentencing laws that require that more of each sentence be served behind bars," he said. Such laws have proliferated at the state and federal levels during the 1990s.

Utah's prison population climbed from 3,972 in 1996 to 4,284 in 1997, an increase of 7.9 percent, tying the state with California, behind Hawaii and Arizona, for the third-highest increase in the West and the 11th-highest in the country.

The year's net gain of 61,186 inmates was very close to the annual average of 63,900 since 1990, when the total population was just under 774,000.

If the decade's average growth rate continues, the combined prison and jail population will top 2 million by 2000, even though crime has been dropping since 1994, according to an estimate by the Sentencing Project, a group that advocates prison alternatives.

The United States trails only Russia in the share of its citizens behind bars. The total U.S. incarceration rate of 645 people per 100,000 is six to 10 times higher than most industrial nations, Sentencing Project said.