Pentagon officials say the move comes in response to an Iranian "campaign" against U.S. interests in the Middle East, blaming the Islamic Republic for a recent attack on tanker vessels at port in the UAE.
The officials spoke on condition of anonymity because the troop plans have not yet been formally announced.
Following reports on Thursday that the Pentagon was preparing plans to deploy as many as 10,000 troops, Shanahan tried to tamp down the appearance that the department was on a war footing, while acknowledging it may need more forces to protect its assets already in the region.
Acting Defense Secretary Patrick Shanahan announced the move Friday to "safeguard USA forces given the ongoing threat posed by Iranian forces".
Also on Friday, the U.S. military accused Iran's Revolutionary Guards (IRGC) of being directly responsible for attacks on tankers off the coast of the United Arab Emirates (UAE) earlier this month, describing it as part of a "campaign" by Tehran driving new USA deployments.
"It may involve sending additional troops", he said.
The new tranche of American forces heading into the Middle East will include a military engineering unit, as well as Air Force fighter squadron, drone units and additional aerial intelligence aircraft, Joint Staff Director Vice Adm. Mike Gilday told reporters at the Pentagon.
But only on Thursday, Mr Trump said that he did not think more troops would be needed. "And I certainly don't think they want to fight with us", Trump said.
A top United States official has accused Iran directly of attacking oil tankers.
"This is truly operations driven by intelligence", Gilday said, adding that the US continues to see intelligence suggesting that Iran is actively planning attacks against the USA and partners in the region by the Revolutionary Guard and Iranian proxies in Yemen and Iraq.
"The additional deployment to the U.S. Central Command area of responsibility is a prudent defensive measure and meant to reduce the possibility of future hostilities", he said.
Earlier this month, the United States sent a carrier attack group and bomber taskforce to the Middle East, citing a "credible threat" from Tehran.
US officials said the Iranian threat to USA forces continues even as Iran has pulled back some weapons systems. Around 1,500 service personnel are likely to be despatched, significantly down on the 5,000-10,000 that some administration sources had floated.
Trump withdrew from the 2015 nuclear deal between Iran and six major powers a year ago and now is seeking to block all Iranian oil exports. Iran has denied involvement in the incidents.
However, the Republican leader played down the potential for military conflict in the region, saying he believed Iran did not want a confrontation with the U.S.