Police in New York are visibly increasing their presence after Tuesday's deadly bombings at Brussels' airport and metro system, authorities told NBC News.

The New York Police Department told NBC it was increasing security at mass transit points, bridges and tunnels and major city landmarks.

That has been a standard NYPD response in past to major terror incidents overseas.

Transit authorities in Washington also indicated, via tweet, that they would step up precautionary patrols Tuesday morning. Extra officers have been deployed and are on the look out for suspicious activity, an Amtrak spokeswoman told CNBC.

Frances Townsend, former Homeland Security adviser to President George W. Bush, said it is much more difficult to enforce spot checks in an open transportation system, such as a metro train or subway, than in a closed system, like an airport. "The challenge there is you increase patrols and sort of random security measures to try and throw off any terrorist who is doing surveillance and trying to launch an attack, but [that is] very, very hard in open systems," she told CNBC's "Squawk Box." Officials have beefed up security lines at airports because they believe the airlines are the targets, but those measures need to be more broadly applied, said Robert Liscouski, former assistant secretary for infrastructure protection at the Department of Homeland Security.