Philadelphia ? FBI statistics Monday confirmed what big cities like Philadelphia, Houston, Cleveland and Las Vegas have seen on the streets: Violent crime in the U.S. is on the rise, posting its biggest one-year increase since 1991.

In Kansas City, Mo., murders rose from 89 in 2004 to 126 in 2005, a 42 percent rise, according to the FBI. Homicides in Philadelphia jumped from 330 to 377, a 14 percent increase. Murders climbed from 272 to 334 in Houston, a 23 percent rise, and from 131 to 144 in Las Vegas, a 10 percent increase.

Jeffrey Sedgwick, director of the U.S. Justice Department’s Bureau of Justice Statistics, cautioned that it is not yet clear whether the FBI numbers reflect a real increase, or the ordinary year-to-year variations that statisticians call “static noise.”

Sedgwick said it is possible that crime rates in the U.S. are approaching a floor below which it may be difficult or even impossible to go. “I’m not sure it’s reasonable to expect you can always drive the crime rate down,” he said.

Some criminal justice experts said the statistics reflect the nation’s complacency in fighting crime. Crime dropped dramatically during the 1990s, and some cities have since abandoned effective programs that emphasized prevention, the putting of more cops on the street, and controls on the spread of guns.

“We see that budgets for policing are being slashed and the federal government has gotten out of that business,” said James Alan Fox, a criminal justice professor at Northeastern University in Boston. Still, Fox said, “We’re still far better off than we were during the double-digit crime inflation we saw in the 1970s.”

Nationally, murders rose 4.8 percent, meaning there were more than 16,900 victims in 2005. That would be the most since 1998 and the largest percentage increase in 15 years.

Some big cities felt the brunt.

Murders rose from 59 to 104 in Birmingham, Ala., up 76 percent; from 59 to 85 in Charlotte-Mecklenburg County, N.C., a 44 percent spike; from 87 to 122 in Milwaukee, a 40 percent jump; and from 79 to 109 in Cleveland, up 38 percent.

In Philadelphia, which has had more than 160 murders this year, the police department has responded by creating a special unit charged with roaming the streets in the dangerous hours between 10 p.m. and 3 a.m. The program, which is expected to start soon, will shift 46 officers from other assignments.

The overall national increase in violent crime was modest, 2.5 percent, which equates to more than 1.4 million crimes. Nevertheless, that was the largest percentage increase since 1991.