Violent crimes in the U.S. increased last year for the first time in six years, mainly due to a spike in crimes in major cities, preliminary data released by the FBI on Monday showed.

Crime rate in the nation rose 1.2 percent in 2012 after declining sharply in previous years since 2006. 

Violent crimes increased 3.7 percent in cities with a population between 500,000 to 1 million and declined by 1 percent in smaller towns, with a population of 10,000 to 24,999 people.

According to a New York Times report, the declining crime rate in the last six years puzzled criminologists, who were inspecting the trend for possible causes of decline.

Some analysts cited by the Times said the recent spike in crime might suggest that the trend is reversing.

“We probably now have answered the question of how low it can go, and we may be bouncing off the bottom now,” Dennis Jay Kenney, a professor at John Jay College of Criminal Justice in New York, told the Times.

But, he added that it may be too early to draw such conclusions. “We probably need another year to tell if we’ve got a pattern here,” he said.

Some analysts pointed out that a depletion in police forces, due to spending cuts, may have led to a surge in the crime rate.

The largest increase in murder and non-negligent manslaughter offenses -- while up in all city groups except one -- was seen in cities with a population between 500,000 to 1 million, shooting up 12.5 percent. On the other hand, cities with a population between 50,000 to 100,000 registered a decline of 1 percent in such offenses, the FBI data showed.

The FBI report is based on preliminary analysis of the annual Uniform Crime Report, or UCR, which is not without its shortcomings, which lead to unexplained swings in statistics from one year to the next, according to a NBC News report.

Many influential leaders view the report as incomplete, the report said, as data for the UCR are provided voluntarily by police and sheriff's departments and state law enforcement agencies.

The UCR data represents 13,770 law enforcement agencies across the country, which means that a third of state and local agencies found in the Justice Department's law enforcement census are yet to find representation in the data.

Besides, a standard reporting mechanism seems to be lacking as participating agencies exercise influence over classification of incidents and charges, the NBC report adds. While some report an event as a single crime regardless of charges, others report each charge separately.

The data also has sparked a debate in the Toledo, Ohio, city council with two members accusing the mayor of manipulating data to political advantage, NBC station WNWO reported

A footnote in the UCR says that data from Toledo was "excluded from all Report tabulations," as it had errors.

Meanwhile, the FBI data showed that violent crime increased in the larger cities -- with a population of more than 1 million -- at a rate of 1.4 percent. Murders were up by 1.5 percent, while rape offenses rose by 3.2 percent. 

Prior to 2012, violence was in decline in the past six years, and the sharpest drop from the previous year, was recorded in 2010 at 6 percent. 

Violent crimes declined by 5.5 percent in 2009 and by 3.8 percent in 2011, while the last surge in such crime was recorded in 2006 when the crime rate rose by 1.9 percent.