Several police officers in the US states of Tennessee, Missouri and Georgia have been shot and wounded in a string of separate attacks against law enforcement amid heightened tensions over the police killing of African Americans in the past week.

The attacks on police, which occurred on Thursday and Friday, come after a gunman in Dallas, Texas, killed five officers and wounded seven more on Thursday night.

The Dallas incident, the deadliest day for police in the US since the September 11, 2001, terror attacks, came during one of several protests across the country against the killing of two black men by police this week.

The fatal shootings of Alton Sterling and Philando Castile in the states of Louisiana and Minnesota were the latest in a long string of killings that gave rise to the Black Lives Matter social movement.

Dallas sniper Micah Xavier Johnson, who was black, said he wanted to “kill white people, especially white officers,” according to the Dallas chief of police. Johnson, 25, ambushed police officers, killing five cops and injuring seven others.

In Bristol Tennessee, a police officer was wounded on Thursday while responding to a public shooting. The Tennessee Bureau of Investigation has identified the suspect as 37-year-old Lakeem Keon Scott.

Early findings indicated Scott may have targeted white people and officers after being troubled by the recent fatal shootings of black people by law enforcement officers in other parts of the country.

In Ballwin, Missouri, a police officer is in critical condition after he was shot in the neck during a traffic stop late Friday morning. In Valdosta, Georgia, reports say a police officer was shot in the line of duty on Friday and is recovering at a local hospital.

The New York Police Department was investigating 17 threats against police that came in following the deaths of two black men in Louisiana and Minnesota on Tuesday and Wednesday, according to NYPD Chief of Intelligence Thomas Galati.

Police in the United States killed over 1,150 people in 2015, with the largest police departments disproportionately killing at least 321 African Americans, according to data compiled by an activist group that runs the Mapping Police Violence project.