The risk is if talks hit a logjam, the U.S. or Israel could decide to strike, potentially prompting retaliatory attacks across the Gulf.

The Pentagon extended the deployment of the USS Harry S. Truman aircraft carrier strike group and ordered a second flotilla to the Middle East, Pentagon Spokesman Sean Parnell confirmed Tuesday. It also sent additional warplanes to the region, including F-35 fighters and B-2 stealth bombers, according to a U.S. official.

A senior Iranian military commander has warned that American troops in the region are vulnerable and "sitting in a glass house."