Violent crime across the U.S. jumped in 2015 after two years of declines, the FBI announced Monday, adding that the number of murders reported by local law enforcement agencies was up 10.8 percent from 2014.

"We still have so much work to do," Attorney General Loretta Lynch responded at an event in Little Rock, Ark. Many major cities, including Chicago, Baltimore and St. Louis, have struggled to restore trust between police and communities in the wake of officer-involved shootings and the Black Lives Matter movement.

Violent crime overall rose by 3.9 percent, the data showed. While up from 2014, it was still down from 2011 and 2006, the FBI reported. The murder rate for U.S. metropolitan areas was slightly higher compared to the rest of the country.

In July, President Obama said, "The fact of the matter is that the murder rate today, the violence rate today, is far lower than it was when Ronald Reagan was president, and lower than when I took office." Since 2009, the nation has seen a 1.9 percent increase in murders, though the murder rate has slipped when population growth is factored in. Total violent crime has fallen by 9.6 percent.

Cities that saw significant crime spikes last year included:

-- Baltimore, where murders and "non-negligent manslaughters" soared from 211 in 2014 to 344 the following year. Violent crime rose 14 percent. The city saw days of riots after the death of Freddie Gray while in police custody, in April 2015.

-- Chicago, where murders jumped by 16 percent and total violent crime rose by 2.4 percent, despite a years-long push to get illegal guns off the streets. In an emotional speech last week, Mayor Rahm Emanuel unveiled a three-part plan to curb gun violence and pleaded for the city to improve its fractured relationship with law enforcement.

-- St. Louis, after the shooting death of Michael Brown led to a string of protests in nearby Ferguson, Mo. Violent crime rose 7 percent in the city of St. Louis in 2015, according to the FBI, and the number of murders jumped 8 percent.

The overall statistics show an estimated 15,696 murders and non-negligent manslaughters in the country in 2015, a 10.8 percent increase from the year before. Those totals do not include killings that agencies have deemed justifiable.

The figures released Monday were reported by law enforcement agencies through the FBI's Uniform Crime Reporting Program, which compiles data on murders, aggravated assault, car thefts and other crime.

"It is important to remember that while crime did increase overall last year, 2015 still represented the third-lowest year for violent crime in the past two decades," Lynch added.

The FBI's crime totals have attracted scrutiny in recent years because of the inconsistent reporting by law enforcement agencies on use of force by police officers.

In a message accompanying this year's report, FBI Director James Comey called for more transparency and accountability in law enforcement and said the FBI is working toward developing a database chronicling incidents of police use of force.

"Information that is accurate, reliable, complete and timely will help all of us learn where we have problems and how to get better," he said.

Fox News' Matt Dean, Courtney Stein Vargas and The Associated Press contributed to this report.