WASHINGTON (AFP) - A US federal probe has concluded that the police in Cleveland - where a 12-year-old boy was recently shot and killed by police - has engaged in a pattern of "using excessive force," Attorney-General Eric Holder announced on Thursday.

The announcement came amid rising racial tensions over a series of police killings of African-Americans, most recently that of Tamir Rice, who was shot to death last month in a Cleveland playground by police responding to reports of a boy brandishing a gun.

Rice was later found to have a toy gun.

"We have determined that there is reasonable cause to believe that the Cleveland Division of Police engages in a pattern or practice of using excessive force - in violation of the Fourth Amendment of the US Constitution, and as a result of systemic deficiencies, including insufficient accountability, inadequate training and equipment, ineffective policies, and inadequate engagement with the community," Holder said.

The federal investigation into police practices in Cleveland was begun 18 months ago and not specifically about the Rice shooting.

But Holder said the city had acknowledged the findings and agreed to take court-enforced remedial actions, including the naming of an independent monitor to oversee reforms.

The action followed a night of protests in New York over a grand jury's decision not to charge a police officer who choked to death an unarmed black man, Eric Garner.

A similar decision on Nov 24 by a grand jury in Missouri in the case of a white officer who killed an unarmed black teen, Michael Brown, sparked riots in the St Louis suburbs and protests in cities across the countries.

The cases have reignited a long-standing debate over police treatment with African-Americans, and what some see as overly aggressive policing tactics.