ABC News' Martha Raddatz and Jonathan Karl report:

On the day that almost two dozen U.S. embassies and consulates across North Africa and the Middle East are closed following the identification of a significant threat from an al-Qaeda affiliate, a senior U.S. official is providing new details about the communications intercepted from the terrorists, telling ABC News that al-Qaeda operatives could be heard talking about an upcoming attack. The official described the terrorists as saying the planned attack is "going to be big" and "strategically significant."

"The part that is alarming is the confidence they showed while communicating and the air of certainty," the official said, adding that the group - Al-Qaeda in the Arabian Peninsula - appeared to have a media plan for after the attack.

Authorities do not know the exact target of the planned attack, according to the official.

"We do not know whether they mean an embassy, an airbase, an aircraft, trains," the official said.

Muhammed Huwais/AFP/Getty Images

Today on "This Week," Rep. Dutch Ruppersberger, D-MD - the ranking member of the House Intelligence Committee - said the intercepted communications called for a "major attack."

"We received information that high level people from al-Qaeda in the Arabian Peninsula are talking about a major attack," Ruppersberger said. "And these are people at a high level."

"It's a very credible threat and it's based on intelligence," Ruppersberger continued. "What we have to do now is the most important issue, is protect Americans throughout the world."

Ruppersberger also commented on the threat's al-Qaeda connection, saying "We know that al-Qaeda and other people out there want to attack us and kill us and our allies."

Read Gen. Martin Dempsey's comments on the latest overseas terror threats here.

The senior U.S. official said there is concern about devices that could be implanted inside the body of a terrorist.

"We are concerned about surgically implanted devices," they said. "These are guys who have developed the techniques to defeat our detection methods."

The official also said authorities were stunned that the group broke "operational security" - meaning they talked likely knowing it would be picked up by intercepts.

ABC News reported Thursday that embassies across the Middle East and North Africa- including those in Egypt, Iraq and Kuwait - would close today because of "a specific threat against a U.S. embassy or consulate."

The next day, the State Department issued a global travel warning to all U.S. citizens around the world, alerting them to the "continued potential for terrorist attacks."

During an interview for "This Week," Joint Chiefs of Staff Chairman Gen. Martin Dempsey told Martha Raddatz that there is "a significant threat stream and we're reacting to it."

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