Department spokesman Brian Roehrkasse said, however, that there were no immediate plans to raise the nation's threat level.
"We do not have any intelligence indicating this type of attack is planned in the United States," Roehrkasse said.
The Homeland Security Department asked authorities in major cities for "continued vigilance" of their transportation systems, Roehrkasse said.
In the nation's capital, bomb-sniffing dogs and armed police officers were patrolling subways and buses and looking for anything suspicious. Passengers were being urged to report any suspicious activity. About 1.2 million people a day ride Washington's buses and trains.
Candace Smith, spokeswoman for the Washington Metropolitan Area Transit Authority, said security in Washington's transit systems was stepped up "immediately" in response to the rush-hour explosions in London.
A police helicopter repeatedly circled the Robert F. Kennedy stadium area, a major transportation hub with several bus stops and large commuter parking lots.
Other major cities also heightened security on transit systems.
In Los Angeles, a police official said police had activated a special command center and officials were meeting to decide whether to upgrade security levels around the city.
A spokeswoman for the Chicago Transit Authority said transit officials were working with Chicago Police on additional security measures.
CTA spokeswoman Kimberly Myles said announcements were being made to riders to be aware of their surroundings and to alert transit workers about suspicious packages.