NEW: No additional security at national monuments in Washington, park police say
New York and San Francisco officials say they're beefing up police presence
City officials emphasize there is no specific threat
It's a response to the concerns that shuttered some U.S. diplomatic sites
As the United States shutters some diplomatic sites around the globe over concerns about a possible attack, at least two major U.S. cities have stepped up their security.
In New York, officials are beefing up police presence, particularly around transportation hubs and religious sites, New York Police Department spokesman James Duffy told CNN.
And in San Francisco, police increased patrols at key locations, including mass transit, popular landmarks and embassies, said spokesman Gordon Shyy.
Officials in both cities emphasized that there was no specific threat leading to the decision. Rather, the moves were made in an abundance of caution.
In Washington, the Metro Transit Police Department sent an e-mail to employees Friday night.
“I want to let you know that at this time, there is no specific or credible threat against Metro, nor any other transportation system in the U.S. However, this worldwide alert serves as an important reminder for us to remain vigilant at all times,” police Chief Ron Pavlik wrote.
U.S. Park Police spokesman Paul Brooks said security has not been increased at national monuments in Washington, and there are no new directives to officers.
In Los Angeles, police and the county sheriff’s department said they had no new formal directives, but were told to be a little more vigilant than usual.
Miami police told CNN it was “business as usual,” and that the department is constantly monitoring for potential threats.
Houston police said they do not comment on “tactical operations.”
CNN’s Josh Levs, Chuck Johnston, Greg Seaby, Chris Boyette, Devon Sayers, Marlena Baldacci and Stella Chan contributed to this report.