The US is moving a Patriot missile defence system from Texas to the Middle East ahead of possible renewed action against Iran, it has been reported.
The deployment comes amid preparations for a potential campaign following the US midterm elections on November 3.
President Donald Trump has not made a final decision on whether to launch further strikes.
The Patriot system includes radar equipment, launchers and interceptor missiles designed to defend against incoming aerial threats.
US forces have also moved troops away from bases considered vulnerable to Iranian attacks and replenished ammunition stocks, officials told the Wall Street Journal.
Washington is said to have warned several Gulf Arab states to prepare for a short, intense military campaign.
Bomber aircraft and fighter jets are also being prepared for possible operations.
An additional aircraft carrier strike group and Marine Expeditionary Unit are expected to join about 20 US warships operating in the region in November, although some vessels may rotate out.
Trump is reportedly considering strikes against Iranian infrastructure and nuclear facilities. A negotiated agreement remains possible but elusive after months of failed attempts to reach a deal.
Iran has threatened to retaliate against energy infrastructure, airports and other civilian facilities across the Persian Gulf if American and Israeli attacks resume.
Tehran has also reportedly instructed Hezbollah to prepare for a possible renewed conflict.
The timing and scope of any further military campaign remain uncertain with Israel not confirming reports.
To get more news, click here to sign up for our free daily newsletter.