WASHINGTON, July 7 (Xinhuanet) -- US Homeland Security Secretary Michael Chertoff announced Thursday to raise the terror alert level from yellow to orange, indicting high risk of terrorist attacks, for mass transit in the wake of the explosions in London.

    "In light of today's attacks in London, the US government is raising the threat level from code yellow or elevated to code orange, high, targeted only to the mass transit portion of the transportation sector," Chertoff said at a news conference.

    "This includes regional and inner-city passenger rail, subways and metropolitan bus systems," he said. "We are also asking for increased vigilance throughout the transportation sector."

A police officer and his dog guard in the lobby hall of the Grand Central Subway Station in New York July 7. (Xinhua)

    Chertoff said there was no specific credible evidence pointing toward an attack in the United States. He stressed that Americans need not to avoid using subways and bus systems, saying there has been "a general elevated level of preparedness all across the country" since Sept. 11, 2001 attacks.

    The increased alert level came hours after four blasts rocked the London subway and tore open a packed double-decker bus during the morning rush hour, killing about 40.

    US President George W. Bush, now in Scotland to attend the G-8 summit meetings, expressed condolences to the victims following the deadly blasts and vowed not to yield to terrorists.

    "We will not yield to the terrorists. We will find them. We will bring them to justice," he said.

    Bush said he has kept in touch with officials of the US Department of Homeland Security and instructed them to be "extra-vigilant." The State Department has told all US embassies to review their security arrangements.

    Security has been stepped up in Washington soon after the London explosions, with bomb-sniffing dogs and armed police officers patrolling subways and buses. A greater number of police officers were deployed around the Pentagon.

    The Federal Bureau of Investigation has sent representatives to London to help investigate the attacks, officials said.

    Chertoff's announcement marked the second time the Homeland Security Department had raised the color-coded level for specific target areas. Alert level was raised for the financial centers in New York, northern New Jersey and Washington D.C. last August following intelligence reports of a threat to major financial institutions such as the International Monetary Fund and the World Bank.

    On the national level, the US government has raised and then lowered back the threat level four times since the warning system was deployed in March 2002. Enditem