Washington - More than 50 US police officers were slain in the line of duty last year, an 89% increase on the number of fatalities in 2013, statistics released by the FBI showed on Monday.A total of 51 officers were killed in 2014, including 46 who died from firearms, compared to 27 in 2013. Despite the increase, the number of fatalities in 2014 is below the average for the period since 1980 when figures first started being compiled by the FBI. Between 1980 and 2014, an average of 64 police officers have died each year.The figures come against a backdrop of mounting nationwide tensions between law enforcement and the communities they police following a series of deaths of unarmed black men which have triggered unrest.FBI director James Comey referenced the "especially challenging relationship between law enforcement and the communities we serve" in a video message to announce the statistics. He called on law enforcement to "do our absolute best to try and see clearly those people we serve and to look for opportunities to have them see us".The statistics came after two police officers were shot and killed during a traffic stop in Mississippi on Saturday. Four men have since been arrested over the slayings.US Attorney General Loretta Lynch decried the Mississippi killings as "shocking" and "appalling"."We will continue to do all that we can to protect our officers across the country and support all those who wear the badge," Lynch added.Of the 2014 death toll, firearms were used in 46 of the 51 police killings. Four officers were killed with vehicles used as weapons and one was killed during a hand-to-hand confrontation.The deaths included 11 fatalities of officers killed while responding to disturbance calls, 10 who died during traffic pursuits or stops and eight killed as a result of ambushes.A further 44 officers died accidentally in the line of duty in 2014, compared with 49 in 2013.