New York, December 23 (RHC)-- Police officers across the United States are on high alert after two policemen in New York City were ambushed by a gunman on Saturday and shot dead. Police departments and their unions around the country warned rank-and-file police officers on Monday to wear bulletproof vests and avoid posting inflammatory statements on social media.

Police were also warned not to make arrests unless absolutely necessary and not patrol alone following the death of New York Police Department officers Rafael Ramos and Wenjian Liu by 28-year-old Ismaaiyl Brinsley.

The NYPD says it has received several threats since the fatal shootings, though there is no evidence any threats are imminent. Brinsley, an African-American, had warned earlier on social media that he would retaliate against the shooting deaths of unarmed African-Americans by white police officers.

The killings come at a time of high tensions over police brutality after several unarmed Black men and young boys were killed by police in recent months. Police nationwide are being condemned for excessive force following the chokehold death of Eric Garner in New York City and the fatal shooting of Michael Brown in Ferguson, Missouri this past summer.

Large-scale protests broke out in recent weeks after grand juries declined to indict the officers involved in those deaths. Based on a study by the Malcolm X Grassroots Movement, 313 African Americans were killed in 2012 by police officers, private security guards and members of the public, and in most cases the perpetrator was not indicted.