(CBS News) In Yemen, large concrete barricades block the road leading to the shuttered U.S. embassy.
It is one of 22 consulates and embassies closed down in a rare move. On Sunday, the State Department announced it is extending the shutdown of those embassies and closing others in the face of a terrorist threat.
The department said it is acting out of "an abundance of caution."
On CBS' "Face the Nation," House Homeland Security Committee Chairman Michael McCaul says the U.S. government is on high alert.
"This is probably one of the most specific and credible threats I've seen, perhaps, since 9/11 and that's why everybody's taking this so seriously," he said.
The intercepts indicate an operation could be underway and may be in the final stages.
Sources tell CBS News the threat appears to be imminent but there are key pieces of information missing with no specifics on time or target location.
That is why broad measures are being taken in addition to embassy and consulate closures. A global travel advisory is currently in place and warns U.S. citizens of the "potential for terrorists to attack public transportation and other tourist infrastructure" such as restaurants and hotels.
Congressman Peter King, R-N.Y., said an attack would most likely happen in the Middle East, but there are no guarantees.
"It could basically be in Europe, it could be in the United States, it could be a series of combined attacks," he said.
Multiple attacks would echo the twin bombings of the U.S. embassies in Kenya and Tanzania 15 years ago this week. More than 200 people died and 4,000 were injured.
President Barack Obama is receiving regular briefings, and there have been high level security meetings at the White House.
That includes the Chairman of the Joint Chiefs of Staff Martin Dempsey.
"There is a significant threat stream and we're reacting to it," he said.
CBS News sources say that the threat level is the most serious it has been since 2006. Even if there is no actual attack carried out in the days ahead, the sources said, the threat will continue into the future until there is clarity.
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