Dec. 23, 2010 -- Just days before Christmas, the White House asked Americans to be vigilant this holiday season, warning of a possible -- though unspecified -- terror threat from Al Qaeda. The caution echoed a weeks' worth of warnings from law enforcement authorities.

A spate of bombs found today in foreign embassies in Rome, Italy has again ratcheted up concern, with authorities calling on people to be on the lookout.

"We remain vigilant to attempts by Al Qaeda and other terrorist organizations to carry out cowardly attacks against innocent men, women, and children, and we are working very closely with other governments to share all threat information immediately and to coordinate closely our counterterrorism and security activities," said John Brennan, the White House counterterrorism chief, said Wednesday in a statement.

The Associated Press cited an unnamed administration official as saying the Homeland Security Department was alerting air carriers to a potential terror tactic involving insulated beverage containers like thermoses. The official stressed that there is no intelligence about an active terror plot.

This week, Brennan convened a meeting of senior law enforcement and intelligence officials to discuss "the latest threat reporting and to coordinate security and counterterrorism plans that will be in place during the holiday season."

Brennan's warning comes days after Attorney General Eric Holder, the FBI, and Department of Homeland Security also alerted the public to a possible attack, citing a year's worth of thwarted attempts starting with last year's Christmas Day "underwear" plot. It was a year ago that a suspected Al Qaeda operative tried to detonate explosives packed in his underwear, onboard a flight bound from Amsterdam to Detroit.

This week Attorney General Eric Holder told ABC News in an exclusive interview that Americans "have to be prepared for potentially bad news."

"What I am trying to do in this interview is to make people aware of the fact that the threat is real, the threat is different, the threat is constant," he said.

Holder said the threat to the U.S. had changed since 9/11, and the country was more likely to suffer an attack at the hands of a homegrown terrorist, like Anwar Al Awlawki, the American-born radical cleric believed to be living in Yemen and directing attacks against the U.S.

Government Bulletin Urges Citizen Caution

In the year since Umar Farouk Abdulmutallab attempted to blow up the Detroit-bound Northwest flight with a bomb concealed in his underwear, authorities have discovered or foiled attempted attacks on Times Square, on cargo planes, and a Christmas tree lighting ceremony in Oregon.

The bulletin warned that terrorists may consider public gatherings like "sporting events, parades, religious and cultural activities" to be attractive targets. "Attacks against these targets could maximize the psychological impact on the American public given the symbolic importance of the holiday season to many in the United States," says the bulletin. "Attacks against air cargo during this busy season are also a concern."

ABC News' Rich Esposito and Pierre Thomas contributed to this report.