The US will deploy an extra 1500 troops to the Middle East to “counter the ongoing threat posed by Iranian forces”, according to acting defence secretary Patrick Shanahan.
President Donald Trump told reporters on Friday the troops would have a “mostly protective” role.
The move comes amid heightened tensions between the USA and Iran, with the Trump administration pointing to intelligence suggesting an imminent threat to US interests in the Middle East.
The forces will number “roughly” 1500 and will deploy in the coming weeks, “with their primary responsibilities and activities being defensive in nature”, according to a copy of the notification obtained by The Associated Press.
Their mission will include protecting US forces already in the region and ensuring freedom of navigation, the notification said.
Officials recently said Pentagon planners had outlined plans that could have sent up to 10,000 military reinforcements to the region. Acting defence secretary Patrick Shanahan later said planners had not settled on a figure.
Washington began reinforcing its presence in the Persian Gulf region this month in response to what it said was a threat from Iran.
The US has tens of thousands of troops in the Middle East, including at a major navy base in Bahrain and an Air Force base and operations centre in Qatar.
There are about 5200 troops in Iraq and 2000 in Syria.
Earlier in May, the US sent thousands more into the region around Iran, including an aircraft carrier strike group, four bomber aircraft and fighter jets.
Mr Trump also stoked public tensions between his country and Iran with a tweet threatening the Islamic Republic with its “official end”.
Mohammad Javad Zarif posted his own message in response on Twitter, saying Trump had been “goaded” into “genocidal taunts”.
Goaded by #B_Team, @realdonaldTrump hopes to achieve what Alexander, Genghis & other aggressors failed to do. Iranians have stood tall for millennia while aggressors all gone. #EconomicTerrorism & genocidal taunts won't "end Iran". #NeverThreatenAnIranian. Try respect—it works!
— Javad Zarif (@JZarif) May 20, 2019
Mr Zarif wrote that Mr Trump “hopes to achieve what Alexander (the Great), Genghis (Khan) & other aggressors failed to do”.
He added: “Iranians have stood tall for a millennia while aggressors all gone.”
He ended his Tweet with #neverthreatenaniranian and: “Try respect – it works!”