Washington ? The Bush administration placed Americans at home and abroad on heightened terrorism alert Tuesday, warning that credible intelligence reports suggest a “high risk” of attacks timed to coincide with the Sept. 11 commemorations.

While President Bush and other administration officials urged citizens to be vigilant but go about their normal lives, the government closed some overseas embassies and increased security at airports, federal buildings and monuments in the face of credible threats against U.S. interests abroad. And Vice President Dick Cheney, who spent much of last fall working away from the president to guarantee presidential succession, canceled an evening appearance and returned to his secret location.

The actions came in response to the decision, approved by Bush, to raise the color-coded terror alert system from yellow, which means significant risk of attack, to orange, the second-highest category on the five-step scale. The nation had been on yellow alert since Homeland Security Director Tom Ridge unveiled the warning system last March.

There was no intelligence suggesting a domestic attack is imminent, the president and other administration officials stressed.

However, Atty. Gen. John Ashcroft and the FBI said, the high level of intercepted “chatter,” intelligence received within the last 24 hours, and refined analysis of earlier intelligence indicates possible car bombings and suicide attacks against U.S. assets in South Asia and the Middle East. And a high-ranking al-Qaida captive, whom U.S. officials refused to name, has indicated that attacks may be unfurled on Sept. 11.

“The threats that we have heard recently remind us of the pattern of threats we heard prior to September the 11th,” Bush said. “We have no specific threat to America, but we’re taking everything seriously, obviously.”

One sign of that was the Pentagon’s last-minute decision to deploy live anti-aircraft missiles to launchers around the nation’s capital for a training exercise. Combat air patrols were already crisscrossing the skies over New York, Washington and elsewhere in preparation for the anniversary of last year’s attacks.

Ashcroft said South Asian al-Qaida cells were thought to be hoarding explosives. The terrorist network’s most likely targets, he said, would be U.S. airliners, other transportation and energy sector targets, embassies, U.S. military bases and national monuments.

In response, the government issued a worldwide caution to Americans traveling overseas, closed more than a dozen diplomatic outposts, stepped up security at military bases and U.S. landmarks, and deployed all available air marshals to the skies. Access to federal facilities was tightened, barriers erected in some places and other precautions taken, Ridge said at a news conference with the attorney general.

The FBI has established command posts at its field offices and dispatched more personnel to a continuously manned operations center.

The U.S. Customs Service tightened security at all of its ports of entry Tuesday and increased examinations of vehicles, cargo, passengers and mail.

Al-Qaida historically hasn’t launched major operations on symbolic dates, with the exception of the thwarted millennium attacks. “September 11th is an important date to us. September 11th is probably not the most critical or important date to a terrorist organization,” a senior FBI official said on condition of anonymity.

Still, Ashcroft said, lower-level al-Qaida operatives could view the date as “a suitable time to lash out in even small strikes to demonstrate their worldwide presence and resolve.”