Homeland Security Secretary Michael Chertoff announced Thursday that the U.S. government was raising the terror alert level to orange (high) but only for mass transit systems, including "regional and inner city passenger rail, subways and metropolitan bus systems."

He said that there is no specific intelligence signaling that attacks are planned on U.S. soil but that the threat level increase is a precautionary measure.

Authorities in major U.S. cities are increasing security, specifically around subway systems, after the deadly attacks on London's transit system. Here are some examples of actions being taken, according to CNN:

Amtrak has deployed more officers and canine teams, while asking the railroad's employees to continue to be vigilant and on alert for suspicious activity.

Transit Police for the Washington Metropolitan Area Transit Authority have deployed special response teams, armed with machine guns, and bomb-sniffing dogs to patrol subway stations, trains and buses. All train station restrooms have been temporarily closed for security reasons.

In Los Angeles, a team of officers and deputy sheriffs will patrol every train, and additional patrols will monitor bus routes.

In New York, daily police staging exercises, which normally focus on the city's financial district, have been refocused on the transit system. Also, thousands of officers have been added or reassigned from other departments for the protection of transit systems.