The weapons include Patriot surface-to-air missiles and a variant of the Navy's SeaRAM and CIWS, or close-in weapon system, which fires 3,000 rounds a minute, a defense official with knowledge of the order told Fox News.
For operational security, the U.S. military is not disclosing what bases are being reinforced.
Two Americans and a British Army medic were killed in a rocket attack outside Baghdad last month, leading to U.S. airstrikes that destroyed five weapons depots used by the Iranian-backed militia blamed for the attack.
Hostilities have escalated in the region between Iran and the United States following a rocket attack in late December 2019 that killed an American contractor in northern Iraq. Following American airstrikes days later on bases in western Iraq and eastern Syria, an angry mob attempted to storm the U.S. Embassy in Baghdad on New Year's Eve.
President Trump ordered the assassination of Iran's most powerful general, Qassem Soleimani, in a drone strike in Baghdad days later. Iran countered by launching more than a dozen ballistic missiles from Iran again bases in Iraq housing American troops.
A series of rocket attacks last month led to U.S. and coalition troops being killed and wounded.
President Trump warned in a tweet Wednesday that Iran "or its proxies" were plotting a "sneak attack" on some of the 5,000 U.S. troops inside Iraq and vowed that if Iran followed through, the Islamic Republic would pay a "very heavy price."
Iran has been ravaged by the coronavirus, and some fear the mounting death toll inside the country, along with the continued U.S. sanctions, could cause Iran to launch another series of attacks.
In recent days, U.S. troops have abandoned three bases in Iraq -- including one where the American contractor was killed -- and turned them over to local forces.
Earlier this week, Soleimani's replacement visited Baghdad, the first known visit from a senior Iranian leader to Iraq's capital since Soleimani was killed, along with Iraqi militia leader Abu Mahdi al-Muhandis.