CHICAGO (Reuters) - Threats against police officers by Chicago gang members angry about the police shooting of an unarmed black teen last month were "unacceptable," Mayor Rahm Emanuel said on Tuesday, as debate over excessive force by law enforcement continues to roil U.S. cities.

On Monday, the Chicago Sun-Times reported that the Chicago Police Department had warned officers that leaders of three gangs had met and plotted to shoot police in response to the fatal shooting of 18-year-old Paul O'Neal on July 28.

The department said it was routine to send alerts when police were threatened, but did not provide further details or confirm that threats had been made by the gangs.

"The idea that a bunch of gang members would threaten violence against the men and women every Chicagoan relies on for their own safety is absolutely unacceptable," Emanuel said in response to the newspaper report.

A string of high-profile killings of black men by police in various U.S. cities in the past two years has renewed a national debate about racial discrimination in the criminal justice system and given rise to the Black Lives Matter movement.

Protests erupted nationwide after the back-to-back killing of black men in Baton Rouge and Minneapolis, but after a rally in Dallas, Texas, a gunman shot dead five police officers in an ambush. Days later, three Baton Rouge police officers were also killed in an ambush.

Tensions over the shooting of O'Neal picked up last week after authorities released videos that captured the moments before and after police shot him.

No firearms were found on O'Neal, who was shot in the back, according to police.

The video footage released on Friday shows two officers firing at a stolen car driven by O'Neal after it sped past them, the car crashing into a police car, and O'Neal running into a backyard where he was shot. The shooting is not shown. It is against departmental policy to fire at or into a moving car when the vehicle was the only potential use of force by a suspect,

(Reporting by Justin Madden; Editing by Sharon Bernstein and Alan Crosby)