Violent crimes unexpectedly jumped 18 percent last year, the first rise in nearly 20 years, and property crimes rose for first time in a decade. But academic experts said the new government data fall short of signaling a reversal of the long decline in crime.

The U.S. Bureau of Justice Statistics reported Wednesday that the increase in the number of violent crimes was the result of an upward swing in simple assaults, which rose 22 percent, from 4 million in 2010 to 5 million last year. The incidence of rape, sexual assault and robbery remained largely unchanged, as did serious violent crime involving weapons or injury.

“2011 may be worse than 2010, but it was also the second-best in recent history,” said Northeastern University criminology professor James Alan Fox.