The Department of Defence confirmed it was sending the Patriot surface-to-air missile system to the region in response to “indications of heightened Iranian readiness to conduct offensive operations against US forces”.
An official said the decision was made after intelligence showed Iran had loaded military equipment and missiles onto small boats controlled by Iran’s Revolutionary Guard (IRGC).
It comes amid rising tensions between the two countries after the US deployed an aircraft carrier to the Middle East this week in response to concerns Iran was planning an attack on American forces or interests in the region.
Tehran responded by announcing it would partially pull out from its commitments to the 2015 international nuclear deal and increase uranium enrichment unless a new agreement is reached in 60 days.
On Wednesday, Donald Trump ordered new sanctions on Iran targeting the country's steel, aluminium, copper and iron sectors, and warned of "further actions unless it [Iran] fundamentally alters its conduct".
A Defence Department official told Associated Press moving the missile system was discussed earlier in the week but it took a few days to get final approval to move the Patriot, a long-range air defence system used to counter tactical ballistic missiles, cruise missiles and advanced aircraft.
The USS Arlington, an amphibious transport ship, will also move to the Middle East earlier than expected, according to the Pentagon.
Iran and the US have each said they are not seeking conflict but both countries have adopted confrontational stances in recent weeks.
In the Pentagon statement, the Defence Department said it was “postured and ready to defend US forces and interests in the region.”
Earlier this week, Yadollah Javani, deputy head of political affairs in the IRGC, said no negotiations would be held with the US and warned “Americans will not dare take military action against us,” according to Iran’s Tasnim News Agency.
When announcing the initial move on Sunday, John Bolton, the national security adviser, cited "troubling and escalatory indications and warnings" from Iran but did not explain what they were.
On Friday, a defence official said threats could include attacks by Iranian proxies, such as Shia militias in Iraq.
US officials told Reuters intelligence indicated Iran had moved missiles onto boats along its shore and an American official suggested missiles were capable of being launched from a small ship.
However, several officials said they have not yet seen any tangible move by Iran in reaction to the US military shifts in the area. They also noted there have been no attacks.
Agencies contributed to this report