U.S. Cities Heighten Security

A Washington Metropolitan Transit police officer walks in a Metro subway station, Thursday morning, July 7, 2005. Security was stepped up in the nation's capital, with bomb-sniffing dogs and armed police officers patrolling subways and buses looking for anything suspicious. Passengers were being urged to report any suspicious activity. (AP Photo/Lauren Victoria Burke)
U.S. counterterror officials tell CBS News that the Homeland Security Department is expected to raise the terror threat level to Orange for rail and subways following the attacks in London. Cities around the country already displayed a heightened state of alert.

President Bush, in Scotland for the G-8 summit, urged vigilance.

The Homeland Security Department says there are no indications of an imminent attack on U.S. interests, a Homeland Security official told CBS News Correspondent Peter Maer.

Other officials were monitoring events in London from the White House Situation Room. A Homeland Security spokeswoman said the department was 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,

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."

At the Pentagon, police officers said they had been placed on a heightened alert shortly after the London attacks.