By Matthew Davis
BBC News, Washington
The United States has raised its terrorist threat warning level from elevated to high across its transport network in the wake of the terror attacks on London.
New York and other major cities have stepped up security
Police with machine guns and sniffer dogs were swiftly deployed on streets and subways in cities like Washington and New York, where the 9/11 terror attacks are still fresh in the memory.
Homeland security chief Michael Chertoff said the security measures were a "commonsense" response to the bombings.
Earlier US President George Bush - at the G8 summit in Scotland - held a video conference with national and homeland security officials, urging them to be "extra vigilant" as the US public headed to work.
Mr Bush - who launched the "war on terror" after 9/11 - expressed "solidarity" with UK Prime Minister Tony Blair and sent "heartfelt
condolences to the people of London".
Security officials said there had been no chatter or intelligence indicating a significant threat to the US and there were no plans for the president to leave the G8 early.
US intelligence continues to believe that the terror threat is greater
outside the US than in it.
But sources predicted an increase in perception of threat in the money
markets and in political and media circles.
Mr Chertoff said he had been "closely monitoring" the London bombings.
"We have been in direct communication with officials at the state and local level and with public and private sector transportation officials," he said.
"We have asked them for increased vigilance and additional security measures for major transit systems.
"We do not have any specific intelligence indicating this type of attack is planned in the United States, but we are constantly evaluating both
intelligence and our protective measures and will take whatever actions are necessary."
The results of the raised threat level would be more police and security on the buses, trains and subways, more spot checks and video surveillance and increased vigilance from staff, he said.
The former Mayor of New York, Rudolph Giuliani, was near London's Liverpool Street station when the blast happened there and described the explosions as "dastardly, cowardly acts".
"My heart goes out to the people who were affected by this, it reminds me so much of September eleven," he said.
Additional precautions were being taken in cities around the US.
In the capital Washington DC, sniffer dogs and special police response teams carrying machine guns were deployed on the underground system.
Toilets were closed at some stations and staff ordered to be extra vigilant.
The US rail operator Amtrak raised its threat level, putting increased
security in place on trains and at stations.
Security around the British embassy in Washington was stepped up in an area already patrolled by the US Secret Service.
There were also increased precautions on New York City's transit system.
Earlier on Thursday morning there had actually been a security drill with armed officers entering the New York subway - although this was unrelated to the London attacks.
As the rush-hour began on the US west coast, Los Angeles police activated a department operation centre and all officers were held over on duty from night shifts.
A US intelligence official told the BBC the attacks were "not
really a surprise".
Public transortation systems were "soft targets" and had been of key
interest for some time, the official added.
US personnel are already on the ground at various sites in London, working with British officials and an FBI team is reported to be on the way to the British capital.
US Secretary of State Condoleezza Rice ordered American embassies around the world to review their security protocols.
Within hours of the explosions, Ms Rice visited the British embassy in Washington to sign a book of condolences, writing "they will not have died in vain".
US Secretary of Defense Donald Rumsfeld said: "We certainly stand with our friend and ally, Great Britain, and we'll be working closely with them in the days and weeks ahead to continue the effort against - the struggle against - extremists in the world."
The US nationwide threat level is currently set at "elevated" - the middle rank of a five-point scale rating the danger of terrorist strikes.