WASHINGTON, July 7: The United States raised the terror alert a notch to code orange for the nation’s mass transit systems on Thursday, responding to a spate of rush-hour bus and subway bombings in London that are believed to have killed at least 33 people.

The heightened alert will apply to “regional and inner city passenger rail, subways and metropolitan bus systems,” but not airlines, Homeland Defence Secretary Michael Chertoff told a news conference in Washington.

In Gleneagles, Scotland, President George Bush vowed to catch those responsible for the bombings as other G8 leaders expressed solidarity with Britain. “We will not yield to these people, will not yield to the terrorists. We will find them, we will bring them to justice, and at the same time, we will spread an ideology of hope and compassion that will overwhelm their ideology of hate,” a posting on the State Department Web site quoted Mr Bush as saying.

In Washington, policemen, armed with automatic weapons and sniffing-dogs were deployed at rail and metro stations and bus terminals. Canine bomb detection teams searched stations, terminals, news kiosks and garbage cans for explosives as morning commuters began to arrive in central Washington.

“We are not suggesting that people avoid public transportation systems. Rather, we are asking that they use those systems, but with an increased awareness of their surroundings,” said Mr Chertoff.

He said that US authorities have “no specific credible evidence” pointing toward an attack in the United States. At the same time, he said, “we are also asking for increased vigilance,” particularly in the US transportation system.

Mr Chertoff told reporters he was not aware of any specific evidence that had foretold the attacks in London several hours earlier but other security experts said US and British officials will now sift through millions of e-mail messages and telephone intercepts to see if there were signs that they had ignored or failed to understand.

FBI announced it is sending a team to London to join the investigation and the US National Intelligence Director John D. Negroponte said another team of intelligence experts would also be sent to London to assist the British.

The terrorism alert level was last raised in August 2004 to orange for the financial services sector in New York City, northern New Jersey and Washington DC, based on intelligence that Al Qaeda was targeting specific buildings. It was lowered back to yellow on Nov 10.

Earlier, President Bush said he had ordered the US Homeland Security Department to contact authorities in major cities and urge them to be “extra vigilant” after the attack in London.

Authorities around the US responded quickly, enhancing security on public transportation systems and urging extra vigilance. In New York, Governor George Pataki urged people to report any suspicious activity or packages, backpacks, boxes to police.

About 4.5 million passengers use New York City subways daily, and officials increased police presence at the subways, buses and transit stations throughout the city.

Officials at Amtrak, which operates trains across North America, said they had increased security at all its stations around the country and posted police officers on board all trains.

In San Francisco, Bay Area Rapid Transit System officials closed all station bathrooms, while Massachusetts Gov. Mitt Romney took the unprecedented step of raising the security level on Boston’s transit system.

In Atlanta, commuter rail police officers and dogs were deployed on metro rails while in Los Angeles police officers were told they couldn’t leave their shifts without permission.