A senior U.S. official told ABC News they believe Al-Qaeda terrorists are planning an attack that is “going to be big” and “strategically significant,” based on intercepted communications.

ABC News reported:

“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.

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

“We are concerned about surgically implanted devices,” 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.

The new details come as 21 U.S. embassies and consulates in the Muslim world closed their doors Sunday in the face of the threat, and a global travel warning was issued for U.S. citizens.

CBS News reported Saturday that a team of operatives planning to carry out a major terror plot was “already in place.”