WASHINGTON -- The number of violent crimes in the United States rose for a second straight year in 2006, marking the first sustained increase in homicides, robberies and other serious offenses since the early 1990s, according to an FBI report to be released Monday.

The FBI's Uniform Crime Report will show an increase of about 1.3 percent in violent offenses last year, including a 6 percent rise in robberies and a slight rise in homicides, according to law enforcement officials. That follows an increase of 2.3 percent in 2005, which was the first significant increase in violent crime in 15 years.

Much of the increase was concentrated in medium-size cities, including the District of Columbia, officials said. Criminologists and law enforcement officials offer varying theories for the upswing, including an increase in the juvenile population, growing numbers of released prison inmates and the rise of serious gang problems in smaller jurisdictions.

Whatever the cause, the statistics are likely to create new political trouble for the Bush administration. Crime fighting has long been a signature Republican political issue, but this week Democratic lawmakers cited the new trend as evidence of weaknesses in federal assistance to local law enforcement.

"After years of driving crime rates down, we're now in reverse gear," said Sen. Joseph R. Biden Jr., D-Del., chairman of a Senate crime subcommittee. "It's time to get back to crime-fighting basics -- that means more cops on the streets, equipped with the tools and resources they need to keep our neighborhoods safe."

With Monday's announcement looming, Attorney General Alberto R. Gonzales unveiled new anti-crime proposals in a speech yesterday at the Bureau of Alcohol, Tobacco, Firearms and Explosives. Mr. Gonzales faces a possible no-confidence vote in the Senate for his handling of the firings of nine U.S. attorneys last year.

"In general, it doesn't appear that the current data reveal nationwide trends," said Mr. Gonzales, who did not disclose specific numbers from the report. "Rather, they show local increases in certain communities. Each community is facing different circumstances, and in many places violent crime continues to decrease."

The statistics do not take into account changes in city populations and reflect only an absolute rise in the number of crimes reported to law enforcement authorities. But Democrats and some law enforcement groups emphasized that the statistics, along with anecdotal reports, show an upward trend in violent crime over the past two years.

These critics say the increase has been spurred by declines in assistance from the federal government, including more than $2 billion in cuts in Justice Department law enforcement programs since 2002.

Many police chiefs and mayors have complained about being stretched particularly thin by counterterrorism demands in the aftermath of the 2001 attacks.