Nearly twice as many black men and women are being imprisoned for drug offenses than are whites, even though studies show that there are five times more white drug users than black ones, an international human rights organization said today.

The report by Human Rights Watch joins a growing body of evidence compiled by liberal advocacy groups showing racial disparities in the country's soaring prison population. That population has quadrupled since 1980 and is expected to surpass two million next year.

''These racial disparities are a national scandal,'' Ken Roth, executive director of Human Rights Watch said. ''Black and white drug offenders get radically different treatment in the American justice system. This is not only profoundly unfair to blacks, it also corrodes the American ideal of equal justice for all.''

The report noted that the overwhelming bulk of the increase in the prison population could be attributed to drug offenses. The report said 62.7 percent of drug offenders sent to prison in 1996, the last year for which complete statistics were available, were African-American, while 36.7 percent were white.

The Census Bureau estimates that blacks currently make up about 12.8 percent of the population and that whites, including Hispanic whites, are about 82.3 percent.

While the Human Rights Watch underlined the racial disparities in the criminal justice system, other experts in the field said the cause was more complicated than racism.

Experts at the Bureau of Criminal Justice Statistics, a division of the Justice Department, say that while studies indicate there are five times as many white users of illegal drugs as black users, drug abuse among African-Americans tends to be more chronic and involve harder drugs such as crack cocaine and heroin.

Indeed, studies suggest black incarceration rates are still being fueled by the crack cocaine epidemic of the 1980's, which helped produce a cadre of chronic drug users, most of whom are above the age of 30. Many of these abusers have either been imprisoned for long sentences or have been in and out of a criminal justice system that fails to provide very much in the way of drug treatment, said Gen. Barry R. McCaffrey, director of the White House Office of National Drug Control Policy.

''Regardless of how severe you wish to be on punishing them you simply have to give them a drug-free prison environment,'' Mr. McCaffrey said. ''And there has to be a follow-on component. That, I would allege is the largest issue.''

In today's report, Human Rights Watch based its study on prison admissions statistics in 1996. States voluntarily provide statistics to the Justice Department, and the study does not include figures for the 13 states that chose not to report statistics.

In much of the country, though, the report paints a picture of stark disparities. In Illinois and Maryland, African-Americans represent 90 percent of those who were incarcerated for selling or using drugs. In five other states -- Louisiana, New Jersey, North Carolina, South Carolina and Virginia -- blacks make up more than 80 percent of those imprisoned on drug offenses.

Researchers from Human Rights Watch point to myriad causes for the disproportionately high number of blacks incarcerated for drugs.

They note that because so much of the drug activity in poor black neighborhoods is conducted on the street, sellers and users are easier to spot and arrest. In addition, they contend that law enforcement agencies direct much of their resources to combating drug activity in black areas, rather than in white neighborhoods where drug sales and use tend to occur behind closed doors.

Also low-income African-Americans often cannot afford lawyers to win them more lenient sentences.

Whatever the reason, Human Rights officials say, the numbers of African-Americans being sent to prison are far out of line with the proportion of blacks and whites shown in studies to use and sell drugs.

And they say that the disparities need to be addressed by politicians, including the two presidential contenders.

''I think racial disparities in the criminal justice system is like the elephant in the room that no one is talking about,'' said Jamie Fellner, associate counsel of Human Rights Watch and the report's author. ''I find it incomprehensible that neither of the presidential candidates is talking about this. There still is a timidity to say anything that would lend them being accused of being soft on crime. Nobody is willing to say enough is enough.''

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