( CNN ) -- 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 .
NEW : No additional security at national monuments in Washington , the police say . New York and San Francisco officials say they 're stepping up their presence . Washington officials emphasize there is no specific threat . It 's a response to the concerns that shuttered some U.S. diplomatic sites .