Philadelphia Police Commissioner Charles H. Ramsey on Saturday night expressed anger over the execution-style killings of two New York City police officers and said his officers needed to be careful and stay alert.

Ramsey, who leads President Obama's Task Force on 21st Century Policing, urged those leading protests of the deaths of Eric Garner on Staten Island and Michael Brown in Ferguson, Mo., to "at least acknowledge the tragedy that occurred tonight and call for calm and not let this escalate any further."

Ramsey said he worries about "the environment that we are in now, with . . . some of the people chanting, 'We want dead cops.' You just don't know who that impacts, and now you have two officers dead, leaving behind families."

John McNesby, president of the local Fraternal Order of Police lodge, agreed that officers "have to be a little extra careful now," adding that the protests give edge to an already dangerous job.

"It's a fragile time," McNesby said. "Cops have to be careful and watch each other's backs."

He said Philadelphia's officers would help New York's department in any way possible: "It's a brotherhood, whether it's Philadelphia, or New York, or whatever. We all stick together."

- Mike Newall and Jonathan Lai