Two U.S. defense officials with direct knowledge of the highly classified message confirmed the details to CNN but said the issue is so sensitive that they could not divulge whose signature was on the message. It was distributed via secure communications in recent days. The officials talked with CNN on condition they not be identified.

Both underscored that the message is not an indication U.S. forces are at any higher state of alert. However, the U.S. military is extremely concerned, they said, that given unrest in Iran, any encounter between U.S. and Iranian units could inadvertently escalate…

One reason for the concern is what the U.S. military views as undisciplined command and control of some Iranian naval units, especially those manned by Iran’s Revolutionary Guard Corps.