americans are on high alert around the world and here at home after a terror threat from al qaeda. At this hour, there are new details about the nature of that threat. Abc news has learned u.S.... See More
americans are on high alert around the world and here at home after a terror threat from al qaeda. At this hour, there are new details about the nature of that threat. Abc news has learned u.S. Intelligence has intercepted communications in which al qaeda says the planned attack is, quote, going to be big, and strategically significant. And this intel -- operatives are in place. 22 embassies across africa and the arab world are shut down, most to be closed for at least a week. Tonight, four countries are added to the list. Security at all of them very high. Security also raised here in the united states at airports and train stations across the country. Our coverage begins with abc's global affairs correspondent, martha raddatz. Reporter: The messages intercepted from al qaeda in the yemen are chilling. A senior u.S. Official telling abc, known terrorists are planning a major attack, which may mean multiple targets aimed at western or u.S. Interest. The official saying the intercepts reveal terrorists talking about an attack that is, quote, going to be big and strategically significant. Officials said the u.S. Believes al qaeda operatives have been selected and are already in place in yemen and possibly additional countries, as well. On abc's "this week," house intelligence committee member dutch ruppersberger told me americans need to take this threat seriously. There's operatives that are in place, because we've received information that high level people from al qaeda in the arabian peninsula are talking about a major attack. Reporter: At the white house, the president called together his entire national security staff for an hours long meeting saturday to coordinate the latest intelligence. The greatest fear, said one official, we are concerned about surgically implanted devices. Bombs placed inside terrorists bodies. A method used before by al qaeda in the arabian peninsula and, quote, these are guys who are have dropped the techniques to defeat our detention methods. This is a wakeup call. Al qaeda, is many ways, stronger than it was before 9/11. Reporter: And the vulnerabilities still clear. A u.S. Official saying, we don't know whether the terrorists are aiming for an embassy, an air base, an aircraft or trains. What officials are hoping is that by talking about this threat and taking precautionary measures, they can avert the strike. But our intelligence assets are now spread far and wide because of the vagueness of the target, which could make the terrorists harder to track. Byron? Martha, thank you.
This transcript has been automatically generated and may not be 100% accurate.