President Bush, in Scotland for the G-8 summit, urged vigilance.
The Homeland Security Department says there are also no indications of an imminent attack on U.S. interests. A Homeland Security official tells CBS News Correspondent Peter Maer the government's "interagency incident management group" is meeting to "assure full situational awareness." Other officials are monitoring events in London from the White House Situation Room. A Homeland Security spokeswoman says the department is in close contact with British intelligence and other agencies.
President Bush had a 10-minute video conference call with National Security advisers back in the U.S., reports CBS News White House Correspondent Mark Knoller.
In a brief speech, Mr. Bush warned Americans to be "extra vigilant" as they head to work after the deadly explosions in London.
"The war on terrorism goes on," he said. "We will not yield to these people. We will not yield to the terrorists."
Terrorism analyst Neil Livingstone tells CBS radio affiliate WTOP in Washington that "it's not a question of if, but when" the U.S. will be attacked again by al Qaeda.
"This is a war that's going to have peaks and valleys," Livingstone said. "Since 9/11, nothing has happened. A lot of Americans think that Osama bin Laden's on the run, therefore we have nothing more to fear."
The explosions on London's transit system prompted heightened security on Washington's subway system. A Metro spokeswoman said officials were "ramping up security immediately," including the use of bomb sniffing dogs. Officers will have a higher visibility, she said, and will be carrying machine guns.
At the Farragut West station, the one closest to the White House, Maer saw transit police using announcements and flashing signs to remind passengers to watch for anything suspicious.