The Pentagon chief said an Iranian attack on an American spy drone in June after Tehran seized a British oil tanker, combined with the attack on two Saudi oil installations last Saturday "represents a dramatic escalation of Iranian aggression."

"To prevent further escalation, Saudi Arabia requested international support to help protect the kingdom's critical infrastructure. The United Arab Emirates has also requested assistance," he said.

"In response to the kingdom's request, the president has approved the deployment of US forces, which will be defensive in nature, and primarily focused on air and missile defense," Esper said.

The deployment to the region would be "moderate," Joint Chiefs of Staff Joe Dunford said, but did not specify the type of equipment or exact number of reinforcements, except that it would be fewer than "thousands of troops.